user_func => sub {
        # worker chunk routine
        my ($mce, $chunk_ref, $chunk_id) = @_;
        my $output = '';

        open my $fh, '<', $chunk_ref;
        while (<$fh>) {
            tr/-!"#%&'()*,.\/:;?@\[\\\]_{}0123456789//d;
            s/w(as|ere)/be/gi;
            s/$RE1/ $W1{lc $1} /g;
            s/$RE2/ $W2{lc $1} /g;
            s/$RE3/ $W3{lc $1} /g;
            $output .= $_;
        }
        close $fh;

        # Output orderly and serially.
        MCE->relay_lock;
        print $OUT_FH $output; $OUT_FH->flush;
        MCE->relay_unlock;
    }