use strict;
use warnings;

my %hash = (a => 1, b => 2, c => 1, d => 3, e => 5);

my @non_unique = duplicated_values_keys(%hash);

print "$_\n" for @non_unique;

sub duplicated_values_keys {
    my %hash = @_;
    my %inverted;
    push @{$inverted{$hash{$_}}}, $_ for keys %hash;
    return map {@{$inverted{$_}} > 1 ? @{$inverted{$_}} : ()} keys %inverted;
}