If you know what the unexpected value is, you can set the watch expression to check the value is not the unexpected one, so it will stay true even for a non-existent value.

w $hash{key} ne "unexpected"
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Note that if the value is changed on the last line of a subroutine, the debugger won't show it, instead it will notice the change the next time you enter the scope.

Example:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

{   my %hash;
    sub set {
        %hash = (a => 12, b => 14);
    }
    sub get {
        my ($key) = @_;
        return $hash{$key}
    }
    sub change {
        $hash{b} = 'Boo';
    }
}

set();

say get('b');

change();

say get('b');
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