use strict;
use warnings;
use Data::Dump::Streamer;

my %dnaLookup = (
    wibble => 'AATCAA 2',
    wobble => 'AATCAA 1',
    plibble => 'GATCAA 3',
    plobble => 'GATCAA 1',
    );
my %reverseLookup;

for my $key (keys %dnaLookup) {
    my ($dna, $loc) = split ' ', $dnaLookup{$key};
    
    push @{$reverseLookup{$dna}}, [$key, $loc];
}

Dump (\%reverseLookup);