use strict;
use warnings;

my %hash = (
    foo => "bar",
    bar => "baz",
    baz => "bar"
);

print join ", ", find_key( \%hash, 'bar' );
print "\n";

sub find_key {
    my ( $hash, $val ) = @_;
    grep { $$hash{$_} eq $val } keys %$hash;
}