A simple tied hash definition would help:

#! perl -slw
use strict;
use 5.010;

{
    package debughash;
    require Tie::Hash;
    use Carp qw[ croak ];
    our @ISA = qw(Tie::Hash);

    sub TIEHASH {
        bless {}, $_[0];
    }

    sub FETCH {
        croak 'Non-existent key' unless exists $_[0]{ $_[1 ] };
        return $_[0]{ $_[1 ] };
    }

    sub STORE {
        return $_[0]{ $_[1 ] } = $_[2];
    }

}

use constant DEBUG => 1;

my %hash;
tie %hash, 'debughash' if DEBUG;

$hash{ 'exists' } = 1;

say $hash{ 'exists' };

say $hash{ 'nonexistant' };

__END__
C:\test>junk51
Non-existent key at C:\test\junk51.pl line 32
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