You can also use exists to distinguish whether hash entries exist. If you don't set a default and the caller does not provide a value, then the entry will not exist.

The following code:

#!/usr/bin/perl
use strict;
use warnings;

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

foreach my $key ('a', 'b', 'c') {
        print "$key " . (defined($hash{$key})?"is":"is not") . " defin
+ed\n";
        print "$key " . (exists($hash{$key})?"does":"does not") . " ex
+ist\n";
}
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produces the following output:

a is defined
a does exist
b is not defined
b does exist
c is not defined
c does not exist
[download]