use strict;
use warnings;

use DBM::Deep;

my $db_filepath = 'lookup.db';
my $file1       = 'XXXX.txt';
my $file2       = 'YYYY.txt';

my $db = DBM::Deep->new($db_filepath);

open(my $fh, $file1) or die "[Error] COULD NOT OPEN FILE [$file1]-[$!]";

while (<$fh>) {

   my $line = $_;

   # get a common key from the data somewhere in here

   my @fields = split (/\s/ ,$line);
   my @output = grep /rs\d{5,}\b/ ,@fields;
   my $rs = join (':' , @output);
   $rs =~ s/:/\n/g;

   $db->{'some-common-key-bewtween-files'} = $rs;

}

close($fh);

# now iterate through your other file and lookup using DBM::Deep

open(my $fh2, $file2) or die "[Error] COULD NOT OPEN FILE [$file2]-[$!]";

while (<$fh2>) {

  my $line = $_;

  if ($line =~ /some-common-key-bewtween-files/) {

     my $db_record = $db->{'some-common-key-bewtween-files'};

     # now you have linked data from both files
     # do your other coding here.

  }

}

close($fh2);