use strict;
use warnings;

use MCE::Loop;
use Data::Dumper;

my @tokens = qw( ab bc cd de ef fg gh );

sub build_regex {
    my ($token) = @_;
    chop $token;
}

MCE::Loop->init(
    chunk_size  => 1,
    max_workers => 4,
);

my %result = mce_loop {
    my $token = $_;
    my $regex = build_regex($token);
    my @line_results = grep { $_ ne $token and /$regex/ } @tokens;
    MCE->gather( $token => \@line_results ) if @line_results;
} @tokens;

MCE::Loop->finish;

print Dumper(\%result), "\n";

__END__

$VAR1 = {
          'ef' => [
                    'fg'
                  ],
          'fg' => [
                    'gh'
                  ],
          'bc' => [
                    'cd'
                  ],
          'ab' => [
                    'bc'
                  ],
          'cd' => [
                    'de'
                  ],
          'de' => [
                    'ef'
                  ]
        };