There's probably a better solution to this, but the coffee isn't holding out for my brain right now:

#!/usr/bin/perl -w

use strict;

my %hash = (
    red    => 2,
    pink   => 1,
    orange => 4,
    black  => 3,
    blue   => 4
);

my %reverse;
while ( my ($k, $v) = each %hash ) {
    push( @{$reverse{$v}}, $k )
}

my @highest = @{ @reverse{
    ( sort { $b <=> $a } keys %reverse )[0]
} };

print join(', ', @highest), $/;
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If you also need to know the value of the keys that have the highest value, here's a little rewrite that also makes the whole operation look simpler:

#!/usr/bin/perl -w

use strict;

my %hash = (
    red    => 2,
    pink   => 1,
    orange => 4,
    black  => 3,
    blue   => 4
);

my %reverse;
while ( my ($k, $v) = each %hash ) {
    push( @{$reverse{$v}}, $k )
}

my $high_val = ( sort { $b <=> $a } keys %reverse )[0];
my @highest = @{ $reverse{$high_val} };

print "highest value: $high_val\n",
    "keys with this value: ", join(', ', @highest), "\n";
[download]