Perl values can be defined or undefined and true or false (all undefined values are also false). Elements in a hash can, in addition, exist or not. The following code might go some way to explaining the differences:

#!/usr/bin/perl -w
use strict;

my %hash = (a => 1,
        b => 0,
        c => undef);

foreach ('a' .. 'd') {
  print "\$hash{$_) : ";

  if (exists $hash{$_}) {
    if (defined $hash{$_}) {
      if ($hash{$_}) {
    print "exists, defined & true\n";
      } else {
    print "exists, defined & false\n";
      }
    } else {
      print "exists, undefined & false\n" 
    }
  } else {
    print "non-existant, undefined & false\n";
  }
}
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<http://www.dave.org.uk>

European Perl Conference - Sept 22/24 2000
<http://www.yapc.org/Europe/>