Other monks have pointed our your source of error. I just want to say that you are doing a lot of unnecessary work. For example, the code

my $value = $list{$location};
$value++;
$list{$location} = $value;
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can be written as -

$list{$location}++;
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And your entire code block can be simplified to just one line of Perl (assuming your $location is never '0') -

$list{ $location || 'Unknown' }++;
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