#!/usr/bin/perl -w
use strict;

my %hash = qw(
    foo 1
    bar 1
    baz 1
);

my $ref = \$hash{bar};

my ($key) = grep { \$hash{$_} eq $ref } keys %hash;
print "$key\n";

# or

my %ref_hash = map { \$hash{$_}, $_ } keys %hash;
print $ref_hash{$ref};