Not a direct answer to your question, but a simpler, threaded solution that might work for you:

#! perl -slw
use strict;
use threads;
use threads::shared;
use Time::HiRes qw[ sleep time ];
use Text::LevenshteinXS qw(distance);

my $minimum_score = 50;
my $max_childs    = 4;

my %contacts :shared;
get_db_data( \%contacts );

print "Starting with max: $max_childs concurrent threads";
my $start = time;

for my $id ( keys %contacts ) {
    sleep 0.1 while threads->list >= $max_childs;

    async( sub {
        my $source_name = shift;
        my $counter = 0;

        for my $contact ( keys %contacts ) {
            my $dest_name = $contacts{ $contact };
            next if $dest_name eq $source_name;

            my $distance = distance( $source_name, $dest_name );
            my $score = 100 - ( $distance * 100 / length( $dest_name )
+ || 1 );

            if( $score >= $minimum_score ) {
                print "$id looks like $contact";
            }
            ++$counter;
        }
        printf "Thread %d compared $id with $counter contacts\n", thre
+ads->tid;
        threads->self->detach;
    }, $contacts{ $id } );
}
sleep 0.1 while threads->list;

printf "Took %6f seconds\n", time() - $start;
exit;

sub get_db_data { ## mock_up
    my $ref = shift;
    $ref->{ $_ } = join '', map{ ( 'A', 'C', 'G', 'T' )[ rand 4 ] } 1 
+.. 1e2
        for 1 .. 25;
    return;
}

__END__
c:\test>866682
Starting with max: 4 concurrent threads
Thread 1 compared 11 with 24 contacts
Thread 2 compared 21 with 24 contacts
Thread 3 compared 7 with 24 contacts
Thread 4 compared 17 with 24 contacts
Thread 5 compared 2 with 24 contacts
22 looks like 1
22 looks like 24
Thread 6 compared 22 with 24 contacts
1 looks like 22
Thread 7 compared 1 with 24 contacts
Thread 8 compared 18 with 24 contacts
Thread 9 compared 23 with 24 contacts
Thread 10 compared 16 with 24 contacts
Thread 11 compared 13 with 24 contacts
Thread 12 compared 25 with 24 contacts
6 looks like 5
Thread 13 compared 6 with 24 contacts
3 looks like 14
Thread 14 compared 3 with 24 contacts
Thread 15 compared 9 with 24 contacts
Thread 16 compared 12 with 24 contacts
Thread 17 compared 20 with 24 contacts
14 looks like 3
Thread 18 compared 14 with 24 contacts
Thread 19 compared 15 with 24 contacts
Thread 20 compared 8 with 24 contacts
Thread 21 compared 4 with 24 contacts
24 looks like 22
Thread 22 compared 24 with 24 contacts
Thread 23 compared 19 with 24 contacts
Thread 24 compared 10 with 24 contacts
5 looks like 6
Thread 25 compared 5 with 24 contacts
Took 0.395703 seconds
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