my $geneids_to_remove = {}; # create a hash reference
$text =~ s/(\w+)
  (?{ # in regex code
    $geneids_to_remove->{$1} = 1; # store geneids in a hash
  })
//gx;


##</code><code>##

my $rx_find_geneid = qr/^(\w+) (?{ $gene_id = $1; })/x;