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#!/usr/bin/perl -w

use strict;
...
my %by_value = reverse %hash;

print Dumper(\%by_value);

$VAR1 = {
          '1' => 'a',
          '2' => 'e'
        };

my %hash = (
  a => 1,
  b => 1,
...
}

print Dumper(\%by_value);

$VAR1 = {
          '1' => [
                   'a',
...
                   'd'
                 ]
        };