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my %hash2 = ();
my %unique = ();
foreach my $test (grep(!$unique{$_}++, values %hash)) {
    $hash2{$test} .= join(",", grep({$hash{$_} eq $test ? hash{$_} : u
+ndef} keys %hash));
}

use strict;
use warnings;
...
                Rate reverse_hash  unique_grep
reverse_hash 28466/s           --          -2%
unique_grep  29104/s           2%           --