perltie

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• NAME
• SYNOPSIS
• DESCRIPTION
  • Tying Scalars
  • Tying Arrays
  • Tying Hashes
  • Tying FileHandles
  • UNTIE this
  • The untie Gotcha
• SEE ALSO
• BUGS
• AUTHOR

NAME

perltie - how to hide an object class in a simple variable

SYNOPSIS

 tie VARIABLE, CLASSNAME, LIST

 $object = tied VARIABLE

 untie VARIABLE

DESCRIPTION

Prior to release 5.0 of Perl, a programmer could use dbmopen() to connect an on-disk database in the standard Unix dbm(3x) format magically to a %HASH in their program. However, their Perl was either built with one particular dbm library or another, but not both, and you couldn't extend this mechanism to other packages or types of variables.

Now you can.

The tie() function binds a variable to a class (package) that will provide the implementation for access methods for that variable. Once this magic has been performed, accessing a tied variable automatically triggers method calls in the proper class. The complexity of the class is hidden behind magic methods calls. The method names are in ALL CAPS, which is a convention that Perl uses to indicate that they're called implicitly rather than explicitly--just like the BEGIN() and END() functions.

In the tie() call, VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of the correct type. Any additional arguments in the LIST are passed to the appropriate constructor method for that class--meaning TIESCALAR(), TIEARRAY(), TIEHASH(), or TIEHANDLE(). (Typically these are arguments such as might be passed to the dbminit() function of C.) The object returned by the "new" method is also returned by the tie() function, which would be useful if you wanted to access other methods in CLASSNAME . (You don't actually have to return a reference to a right "type" (e.g., HASH or CLASSNAME ) so long as it's a properly blessed object.) You can also retrieve a reference to the underlying object using the tied() function.

Unlike dbmopen(), the tie() function will not use or require a module for you--you need to do that explicitly yourself.

Tying Scalars

A class implementing a tied scalar should define the following methods: TIESCALAR, FETCH, STORE, and possibly UNTIE and/or DESTROY.

Let's look at each in turn, using as an example a tie class for scalars that allows the user to do something like:

    tie $his_speed, 'Nice', getppid();
    tie $my_speed,  'Nice', $$;

And now whenever either of those variables is accessed, its current system priority is retrieved and returned. If those variables are set, then the process's priority is changed!

We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource class (not included) to access the PRIO_PROCESS, PRIO_MIN, and PRIO_MAX constants from your system, as well as the getpriority() and setpriority() system calls. Here's the preamble of the class.

    package Nice;
    use Carp;
    use BSD::Resource;
    use strict;
    $Nice::DEBUG = 0 unless defined $Nice::DEBUG;

• TIESCALAR classname, LIST

  This is the constructor for the class. That means it is expected to return a blessed reference to a new scalar (probably anonymous) that it's creating. For example:

      sub TIESCALAR {
          my $class = shift;
          my $pid = shift || $$; # 0 means me

          if ($pid !~ /^\d+$/) {
              carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
              return undef;
          }

          unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
              carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
              return undef;
          }

          return bless \$pid, $class;
      }

  This tie class has chosen to return an error rather than raising an exception if its constructor should fail. While this is how dbmopen() works, other classes may well not wish to be so forgiving. It checks the global variable $^W to see whether to emit a bit of noise anyway.

• FETCH this

  This method will be triggered every time the tied variable is accessed (read). It takes no arguments beyond its self reference, which is the object representing the scalar we're dealing with. Because in this case we're using just a SCALAR ref for the tied scalar object, a simple $$self allows the method to get at the real value stored there. In our example below, that real value is the process ID to which we've tied our variable.

      sub FETCH {
          my $self = shift;
          confess "wrong type" unless ref $self;
          croak "usage error" if @_;
          my $nicety;
          local($!) = 0;
          $nicety = getpriority(PRIO_PROCESS, $$self);
          if ($!) { croak "getpriority failed: $!" }
          return $nicety;
      }

  This time we've decided to blow up (raise an exception) if the renice fails--there's no place for us to return an error otherwise, and it's probably the right thing to do.

• STORE this, value

  This method will be triggered every time the tied variable is set (assigned). Beyond its self reference, it also expects one (and only one) argument--the new value the user is trying to assign. Don't worry about returning a value from STORE -- the semantic of assignment returning the assigned value is implemented with FETCH.

      sub STORE {
          my $self = shift;
          confess "wrong type" unless ref $self;
          my $new_nicety = shift;
          croak "usage error" if @_;

          if ($new_nicety < PRIO_MIN) {
              carp sprintf
                "WARNING: priority %d less than minimum system priority %d",
                    $new_nicety, PRIO_MIN if $^W;
              $new_nicety = PRIO_MIN;
          }

          if ($new_nicety > PRIO_MAX) {
              carp sprintf
                "WARNING: priority %d greater than maximum system priority %d",
                    $new_nicety, PRIO_MAX if $^W;
              $new_nicety = PRIO_MAX;
          }

          unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
              confess "setpriority failed: $!";
          }
      }

• UNTIE this

  This method will be triggered when the untie occurs. This can be useful if the class needs to know when no further calls will be made. (Except DESTROY of course.) See "untie.html">untie Gotcha" in The DESTROY this

  This method will be triggered when the tied variable needs to be destructed. As with other object classes, such a method is seldom necessary, because Perl deallocates its moribund object's memory for you automatically--this isn't C++, you know. We'll use a DESTROY method here for debugging purposes only.

      sub DESTROY {
          my $self = shift;
          confess "wrong type" unless ref $self;
          carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
      }

    use FixedElem_Array;
    tie @array, 'FixedElem_Array', 3;
    $array[0] = 'cat';  # ok.
    $array[1] = 'dogs'; # exception, length('dogs') > 3.

    package FixedElem_Array;
    use Carp;
    use strict;

• TIEARRAY classname, LIST

  This is the constructor for the class. That means it is expected to return a blessed reference through which the new array (probably an anonymous ARRAY ref) will be accessed.

  In our example, just to show you that you don't really have to return an ARRAY reference, we'll choose a HASH reference to represent our object. A HASH works out well as a generic record type: the {ELEMSIZE} field will store the maximum element size allowed, and the {ARRAY} field will hold the true ARRAY ref. If someone outside the class tries to dereference the object returned (doubtless thinking it an ARRAY ref), they'll blow up. This just goes to show you that you should respect an object's privacy.

      sub TIEARRAY {
        my $class    = shift;
        my $elemsize = shift;
        if ( @_ || $elemsize =~ /\D/ ) {
          croak "usage: tie ARRAY, '" . __PACKAGE__ . "', elem_size";
        }
        return bless {
          ELEMSIZE => $elemsize,
          ARRAY    => [],
        }, $class;
      }

• FETCH this, index

  This method will be triggered every time an individual element the tied array is accessed (read). It takes one argument beyond its self reference: the index whose value we're trying to fetch.

      sub FETCH {
        my $self  = shift;
        my $index = shift;
        return $self->{ARRAY}->[$index];
      }

  If a negative array index is used to read from an array, the index will be translated to a positive one internally by calling FETCHSIZE before being passed to FETCH. You may disable this feature by assigning a true value to the variable $NEGATIVE_INDICES in the tied array class.

  As you may have noticed, the name of the FETCH method (et al.) is the same for all accesses, even though the constructors differ in names (TIESCALAR vs TIEARRAY). While in theory you could have the same class servicing several tied types, in practice this becomes cumbersome, and it's easiest to keep them at simply one tie type per class.

• STORE this, index, value

  This method will be triggered every time an element in the tied array is set (written). It takes two arguments beyond its self reference: the index at which we're trying to store something and the value we're trying to put there.

  In our example, undef is really $self->{ELEMSIZE} number of spaces so we have a little more work to do here:

      sub STORE {
        my $self = shift;
        my( $index, $value ) = @_;
        if ( length $value > $self->{ELEMSIZE} ) {
          croak "length of $value is greater than $self->{ELEMSIZE}";
        }
        # fill in the blanks
        $self->EXTEND( $index ) if $index > $self->FETCHSIZE();
        # right justify to keep element size for smaller elements
        $self->{ARRAY}->[$index] = sprintf "%$self->{ELEMSIZE}s", $value;
      }

  Negative indexes are treated the same as with FETCH.

• FETCHSIZE this

  Returns the total number of items in the tied array associated with object this. (Equivalent to scalar(@array)). For example:

      sub FETCHSIZE {
        my $self = shift;
        return scalar @{$self->{ARRAY}};
      }

• STORESIZE this, count

  Sets the total number of items in the tied array associated with object this to be count. If this makes the array larger then class's mapping of undef should be returned for new positions. If the array becomes smaller then entries beyond count should be deleted.

  In our example, 'undef' is really an element containing $self->{ELEMSIZE} number of spaces. Observe:

      sub STORESIZE {
        my $self  = shift;
        my $count = shift;
        if ( $count > $self->FETCHSIZE() ) {
          foreach ( $count - $self->FETCHSIZE() .. $count ) {
            $self->STORE( $_, '' );
          }
        } elsif ( $count < $self->FETCHSIZE() ) {
          foreach ( 0 .. $self->FETCHSIZE() - $count - 2 ) {
            $self->POP();
          }
        }
      }

• EXTEND this, count

  Informative call that array is likely to grow to have count entries. Can be used to optimize allocation. This method need do nothing.

  In our example, we want to make sure there are no blank (undef) entries, so EXTEND will make use of STORESIZE to fill elements as needed:

      sub EXTEND {   
        my $self  = shift;
        my $count = shift;
        $self->STORESIZE( $count );
      }

• EXISTS this, key

  Verify that the element at index key exists in the tied array this.

  In our example, we will determine that if an element consists of $self->{ELEMSIZE} spaces only, it does not exist:

      sub EXISTS {
        my $self  = shift;
        my $index = shift;
        return 0 if ! defined $self->{ARRAY}->[$index] ||
                    $self->{ARRAY}->[$index] eq ' ' x $self->{ELEMSIZE};
        return 1;
      }

• DELETE this, key

  Delete the element at index key from the tied array this.

  In our example, a deleted item is $self->{ELEMSIZE} spaces:

      sub DELETE {
        my $self  = shift;
        my $index = shift;
        return $self->STORE( $index, '' );
      }

• CLEAR this

  Clear (remove, delete, ...) all values from the tied array associated with object this. For example:

      sub CLEAR {
        my $self = shift;
        return $self->{ARRAY} = [];
      }

• PUSH this, LIST

  Append elements of LIST to the array. For example:

      sub PUSH {  
        my $self = shift;
        my @list = @_;
        my $last = $self->FETCHSIZE();
        $self->STORE( $last + $_, $list[$_] ) foreach 0 .. $#list;
        return $self->FETCHSIZE();
      }

• POP this

  Remove last element of the array and return it. For example:

      sub POP {
        my $self = shift;
        return pop @{$self->{ARRAY}};
      }

• SHIFT this

  Remove the first element of the array (shifting other elements down) and return it. For example:

      sub SHIFT {
        my $self = shift;
        return shift @{$self->{ARRAY}};
      }

• UNSHIFT this, LIST

  Insert LIST elements at the beginning of the array, moving existing elements up to make room. For example:

      sub UNSHIFT {
        my $self = shift;
        my @list = @_;
        my $size = scalar( @list );
        # make room for our list
        @{$self->{ARRAY}}[ $size .. $#{$self->{ARRAY}} + $size ]
         = @{$self->{ARRAY}};
        $self->STORE( $_, $list[$_] ) foreach 0 .. $#list;
      }

• SPLICE this, offset, length, LIST

  Perform the equivalent of splice on the array.

  offset is optional and defaults to zero, negative values count back from the end of the array.

  length is optional and defaults to rest of the array.

  LIST may be empty.

  Returns a list of the original length elements at offset.

  In our example, we'll use a little shortcut if there is a LIST:

      sub SPLICE {
        my $self   = shift;
        my $offset = shift || 0;
        my $length = shift || $self->FETCHSIZE() - $offset;
        my @list   = (); 
        if ( @_ ) {
          tie @list, __PACKAGE__, $self->{ELEMSIZE};
          @list   = @_;
        }
        return splice @{$self->{ARRAY}}, $offset, $length, @list;
      }

• UNTIE this

  Will be called when untie happens. (See "untie.html">untie Gotcha" in The DESTROY this

  This method will be triggered when the tied variable needs to be destructed. As with the scalar tie class, this is almost never needed in a language that does its own garbage collection, so this time we'll just leave it out.

    use DotFiles;
    tie %dot, 'DotFiles';
    if ( $dot{profile} =~ /MANPATH/ ||
         $dot{login}   =~ /MANPATH/ ||
         $dot{cshrc}   =~ /MANPATH/    )
    {
	print "you seem to set your MANPATH\n";
    }

    tie %him, 'DotFiles', 'daemon';
    foreach $f ( keys %him ) {
	printf "daemon dot file %s is size %d\n",
	    $f, length $him{$f};
    }

    package DotFiles;
    use Carp;
    sub whowasi { (caller(1))[3] . '()' }
    my $DEBUG = 0;
    sub debug { $DEBUG = @_ ? shift : 1 }

• TIEHASH classname, LIST

  This is the constructor for the class. That means it is expected to return a blessed reference through which the new object (probably but not necessarily an anonymous hash) will be accessed.

  Here's the constructor:

      sub TIEHASH {
  	my $self = shift;
  	my $user = shift || $>;
  	my $dotdir = shift || '';
  	croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
  	$user = getpwuid($user) if $user =~ /^\d+$/;
  	my $dir = (getpwnam($user))[7]
  		|| croak "@{[&whowasi]}: no user $user";
  	$dir .= "/$dotdir" if $dotdir;

  	my $node = {
  	    USER    => $user,
  	    HOME    => $dir,
  	    LIST    => {},
  	    CLOBBER => 0,
  	};

  	opendir(DIR, $dir)
  		|| croak "@{[&whowasi]}: can't opendir $dir: $!";
  	foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
  	    $dot =~ s/^\.//;
  	    $node->{LIST}{$dot} = undef;
  	}
  	closedir DIR;
  	return bless $node, $self;
      }

  It's probably worth mentioning that if you're going to filetest the return values out of a readdir, you'd better prepend the directory in question. Otherwise, because we didn't chdir() there, it would have been testing the wrong file.

• FETCH this, key

  This method will be triggered every time an element in the tied hash is accessed (read). It takes one argument beyond its self reference: the key whose value we're trying to fetch.

  Here's the fetch for our DotFiles example.

      sub FETCH {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	my $dot = shift;
  	my $dir = $self->{HOME};
  	my $file = "$dir/.$dot";

  	unless (exists $self->{LIST}->{$dot} || -f $file) {
  	    carp "@{[&whowasi]}: no $dot file" if $DEBUG;
  	    return undef;
  	}

  	if (defined $self->{LIST}->{$dot}) {
  	    return $self->{LIST}->{$dot};
  	} else {
  	    return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
  	}
      }

  It was easy to write by having it call the Unix cat(1) command, but it would probably be more portable to open the file manually (and somewhat more efficient). Of course, because dot files are a Unixy concept, we're not that concerned.

• STORE this, key, value

  This method will be triggered every time an element in the tied hash is set (written). It takes two arguments beyond its self reference: the index at which we're trying to store something, and the value we're trying to put there.

  Here in our DotFiles example, we'll be careful not to let them try to overwrite the file unless they've called the clobber() method on the original object reference returned by tie().

      sub STORE {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	my $dot = shift;
  	my $value = shift;
  	my $file = $self->{HOME} . "/.$dot";
  	my $user = $self->{USER};

  	croak "@{[&whowasi]}: $file not clobberable"
  	    unless $self->{CLOBBER};

  	open(F, "> $file") || croak "can't open $file: $!";
  	print F $value;
  	close(F);
      }

  If they wanted to clobber something, they might say:

      $ob = tie %daemon_dots, 'daemon';
      $ob->clobber(1);
      $daemon_dots{signature} = "A true daemon\n";

  Another way to lay hands on a reference to the underlying object is to use the tied() function, so they might alternately have set clobber using:

      tie %daemon_dots, 'daemon';
      tied(%daemon_dots)->clobber(1);

  The clobber method is simply:

      sub clobber {
  	my $self = shift;
  	$self->{CLOBBER} = @_ ? shift : 1;
      }

• DELETE this, key

  This method is triggered when we remove an element from the hash, typically by using the delete() function. Again, we'll be careful to check whether they really want to clobber files.

      sub DELETE   {
  	carp &whowasi if $DEBUG;

  	my $self = shift;
  	my $dot = shift;
  	my $file = $self->{HOME} . "/.$dot";
  	croak "@{[&whowasi]}: won't remove file $file"
  	    unless $self->{CLOBBER};
  	delete $self->{LIST}->{$dot};
  	my $success = unlink($file);
  	carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
  	$success;
      }

  The value returned by DELETE becomes the return value of the call to delete(). If you want to emulate the normal behavior of delete(), you should return whatever FETCH would have returned for this key. In this example, we have chosen instead to return a value which tells the caller whether the file was successfully deleted.

• CLEAR this

  This method is triggered when the whole hash is to be cleared, usually by assigning the empty list to it.

  In our example, that would remove all the user's dot files! It's such a dangerous thing that they'll have to set CLOBBER to something higher than 1 to make it happen.

      sub CLEAR    {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
  	    unless $self->{CLOBBER} > 1;
  	my $dot;
  	foreach $dot ( keys %{$self->{LIST}}) {
  	    $self->DELETE($dot);
  	}
      }

• EXISTS this, key

  This method is triggered when the user uses the exists() function on a particular hash. In our example, we'll look at the {LIST} hash element for this:

      sub EXISTS   {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	my $dot = shift;
  	return exists $self->{LIST}->{$dot};
      }

• FIRSTKEY this

  This method will be triggered when the user is going to iterate through the hash, such as via a keys() or each() call.

      sub FIRSTKEY {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	my $a = keys %{$self->{LIST}};		# reset each() iterator
  	each %{$self->{LIST}}
      }

• NEXTKEY this, lastkey

  This method gets triggered during a keys() or each() iteration. It has a second argument which is the last key that had been accessed. This is useful if you're carrying about ordering or calling the iterator from more than one sequence, or not really storing things in a hash anywhere.

  For our example, we're using a real hash so we'll do just the simple thing, but we'll have to go through the LIST field indirectly.

      sub NEXTKEY  {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	return each %{ $self->{LIST} }
      }

• SCALAR this

  This is called when the hash is evaluated in scalar context. In order to mimic the behaviour of untied hashes, this method should return a false value when the tied hash is considered empty. If this method does not exist, perl will make some educated guesses and return true when the hash is inside an iteration. If this isn't the case, FIRSTKEY is called, and the result will be a false value if FIRSTKEY returns the empty list, true otherwise.

  However, you should not blindly rely on perl always doing the right thing. Particularly, perl will mistakenly return true when you clear the hash by repeatedly calling DELETE until it is empty. You are therefore advised to supply your own SCALAR method when you want to be absolutely sure that your hash behaves nicely in scalar context.

  In our example we can just call scalar on the underlying hash referenced by $self->{LIST} :

      sub SCALAR {
  	carp &whowasi if $DEBUG;
  	my $self = shift;
  	return scalar %{ $self->{LIST} }
      }

• UNTIE this

  This is called when untie occurs. See "untie.html">untie Gotcha" in The DESTROY this

  This method is triggered when a tied hash is about to go out of scope. You don't really need it unless you're trying to add debugging or have auxiliary state to clean up. Here's a very simple function:

      sub DESTROY  {
  	carp &whowasi if $DEBUG;
      }

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

    package Shout;

• TIEHANDLE classname, LIST

  This is the constructor for the class. That means it is expected to return a blessed reference of some sort. The reference can be used to hold some internal information.

      sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }

• WRITE this, LIST

  This method will be called when the handle is written to via the syswrite function.

      sub WRITE {
  	$r = shift;
  	my($buf,$len,$offset) = @_;
  	print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
      }

• PRINT this, LIST

  This method will be triggered every time the tied handle is printed to with the print() function. Beyond its self reference it also expects the list that was passed to the print function.

      sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }

• PRINTF this, LIST

  This method will be triggered every time the tied handle is printed to with the printf() function. Beyond its self reference it also expects the format and list that was passed to the printf function.

      sub PRINTF {
          shift;
          my $fmt = shift;
          print sprintf($fmt, @_);
      }

• READ this, LIST

  This method will be called when the handle is read from via the read or sysread functions.

      sub READ {
  	my $self = shift;
  	my $bufref = \$_[0];
  	my(undef,$len,$offset) = @_;
  	print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
  	# add to $$bufref, set $len to number of characters read
  	$len;
      }

• READLINE this

  This method will be called when the handle is read from via <HANDLE>. The method should return undef when there is no more data.

      sub READLINE { $r = shift; "READLINE called $$r times\n"; }

• GETC this

  This method will be called when the getc function is called.

      sub GETC { print "Don't GETC, Get Perl"; return "a"; }

• CLOSE this

  This method will be called when the handle is closed via the close function.

      sub CLOSE { print "CLOSE called.\n" }

• UNTIE this

  As with the other types of ties, this method will be called when untie happens. It may be appropriate to "auto CLOSE" when this occurs. See "untie.html">untie Gotcha" in The DESTROY this

  As with the other types of ties, this method will be called when the tied handle is about to be destroyed. This is useful for debugging and possibly cleaning up.

      sub DESTROY { print "</shout>\n" }

    tie(*FOO,'Shout');
    print FOO "hello\n";
    $a = 4; $b = 6;
    print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
    print <FOO>;

You can define for all tie types an UNTIE method that will be called at untie(). See "untie.html">untie Gotcha" in The

The untie Gotcha

    package Remember;

    use strict;
    use warnings;
    use IO::File;

    sub TIESCALAR {
        my $class = shift;
        my $filename = shift;
        my $handle = IO::File->new( "> $filename" )
                         or die "Cannot open $filename: $!\n";

        print $handle "The Start\n";
        bless {FH => $handle, Value => 0}, $class;
    }

    sub FETCH {
        my $self = shift;
        return $self->{Value};
    }

    sub STORE {
        my $self = shift;
        my $value = shift;
        my $handle = $self->{FH};
        print $handle "$value\n";
        $self->{Value} = $value;
    }

    sub DESTROY {
        my $self = shift;
        my $handle = $self->{FH};
        print $handle "The End\n";
        close $handle;
    }

    1;

    use strict;
    use Remember;

    my $fred;
    tie $fred, 'Remember', 'myfile.txt';
    $fred = 1;
    $fred = 4;
    $fred = 5;
    untie $fred;
    system "cat myfile.txt";

    The Start
    1
    4
    5
    The End

    sub comment {
        my $self = shift;
        my $text = shift;
        my $handle = $self->{FH};
        print $handle $text, "\n";
    }

    use strict;
    use Remember;

    my ($fred, $x);
    $x = tie $fred, 'Remember', 'myfile.txt';
    $fred = 1;
    $fred = 4;
    comment $x "changing...";
    $fred = 5;
    untie $fred;
    system "cat myfile.txt";

    untie attempted while 1 inner references still exist

    undef $x;
    untie $fred;

    sub UNTIE
    {
     my ($obj,$count) = @_;
     carp "untie attempted while $count inner references still exist" if $count;
    }