use strict;
use warnings;
use Mmap;
open my $file, '<', 'big21gfilename.ext' or die;
mmap my $f, 0, PROT_READ, MAP_SHARED, $file or die;
my $re = '('+join('|',@ARGV)+')';
$re = qr($re);
printf "%d: %s\n", pos($f), $1 while $f =~ /$re/g;