use strict;
use warnings;
use feature 'say';
my %hash = ( 'a' => 1, 'b' => 2, 'c' => '1', 'd' => 3,
             'e' => '5');
  my (%seen,@dups); 
  while (my($k,$v)=each %hash){
    if ( defined $seen{$v}){
      push @dups,$k;
      $seen{$v} and push @dups,$seen{$v};
      $seen{$v}='';
      next
    }
    $seen{$v}=$k
  }
say for sort @dups;
__OUTPUT__
a
c
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