$ perl -e '
    use Data::Dump;
    my %h = qw{cat Tibbles dog Rover kitten Tibbles};
    my %r = reverse %h;
    dd \%h;
    dd \%r;
'
{ cat => "Tibbles", dog => "Rover", kitten => "Tibbles" }
{ Rover => "dog", Tibbles => "cat" }

$ perl -e '
    use Data::Dump;
    my %h = qw{cat Tibbles dog Rover kitten Tibbles};
    my %r = reverse %h;
    dd \%h;
    dd \%r;
'
{ cat => "Tibbles", dog => "Rover", kitten => "Tibbles" }
{ Rover => "dog", Tibbles => "kitten" }

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Benchmark 'cmpthese';
use List::Util 'pairmap';
use Data::Dump;

my %h = qw{mammal cat mollusc snail fish cat};

print "*** Check initial data\n";
dd \%h;

print "*** Check subs return the same value\n";
print "_for\n";
dd _for();
print "_map\n";
dd _map();
print "_pairmap\n";
dd _pairmap();

print "*** Benchmark\n";
cmpthese 0 => {
    for => \&_for,
    map => \&_map,
    pairmap => \&_pairmap,
};

sub _for {
    my $result = [];

    for my $key (keys %h) {
        next unless $h{$key} eq 'cat';
        push @$result, $h{$key}, $key;
    }

    return $result;
}

sub _map {
    my $result = [];

    @$result = map { $h{$_} eq 'cat' ? ($h{$_}, $_) : () } keys %h;

    return $result;
}

sub _pairmap {
    my $result = [];

    @$result = pairmap { $b eq 'cat' ? ($b, $a) : () } %h;

    return $result;
}

##</code><code>##

*** Check initial data
{ fish => "cat", mammal => "cat", mollusc => "snail" }
*** Check subs return the same value
_for
["cat", "fish", "cat", "mammal"]
_map
["cat", "fish", "cat", "mammal"]
_pairmap
["cat", "fish", "cat", "mammal"]

##</code><code>##

*** Benchmark
             Rate pairmap     map     for
pairmap  775527/s      --    -33%    -42%
map     1154992/s     49%      --    -14%
for     1346695/s     74%     17%      --

*** Benchmark
             Rate pairmap     map     for
pairmap  779518/s      --    -34%    -43%
map     1177598/s     51%      --    -13%
for     1360223/s     74%     16%      --

*** Benchmark
             Rate pairmap     map     for
pairmap  781507/s      --    -34%    -43%
map     1178381/s     51%      --    -14%
for     1364313/s     75%     16%      --