How many cpus have you? Neither threads nor processes are likely to help unless you have more than one, and even then it's unlikely to get much quicker using the method you are considering as they would all be accessing the same disc and file. You would likely just slow things down.

One thing that might help is to load the file as a single string. I know you said you do not have enough memory, but I am guessing that you have been trying to load the file as an array of lines which takes a lot more ram than if you load it as a single string. It's a rare machine these days that does not have 200MB available.

The following shows my fairly average machine loading a 200MB file consisting of 100-character lines of random digits and then searching the resultant string and finding 2000+ occurances all in just under 3/4 of a second.

#! perl -slw
use strict;
use Benchmark::Timer; my $T = new Benchmark::Timer;

$T->start('read');
open IN, '<', $ARGV[ 0 ] or die $!;
my $text;
sysread IN, $text, -s( $ARGV[ 0 ] ) or die $!;
$T->stop('read');
printf "file contains %d bytes\n", length $text;

my $count = 0;
$T->start('regex');
$count++ while $text =~ m[(12345)]g;
$T->stop('regex');
print "$count matches found";

$T->report;

__END__
C:\test>junk junk.dat
file contains 213909504 bytes
2016 matches found
1 trial of read (452.624ms total)

1 trial of regex (260.872ms total)
[download]

If you are searching for multiple texts and need to capture/output whole lines then things will be slower, but it's not possible to be more realistic without better information.