Note this is extremely large file and can't put the whole file into a string.

How large is the section you want to read, does that fit into memory? One possible approach is to buffer the lines in an array, as several monks have shown. Just for fun, I thought about what a script like this might do if you didn't want to read the whole file nor the section being searched for into memory; in that case you could scan the file and remember the byte offsets of the strings you're looking for. In the following, I'm reading that data in, but that's not required, you could do something else with those byte offsets. Note that the code below only works with bytes, not Unicode characters.

use warnings;
use strict;

my $file = 'in.txt';
my ($start,$end);
open my $fh, '<:raw', $file or die "$file: $!";
my $offset = 0;
while (<$fh>) {
    $start = $offset if /PATTERN1/;
    $offset = tell $fh or die "tell: $!";
    $end = $offset if /PATTERN3/;
}
die "Failed to find second pattern after first pattern"
    unless defined $start && defined $end && $end > $start;
seek $fh, $start, 0 or die "seek: $!";
my $bytes = $end-$start;
read($fh, my $data, $bytes)==$bytes
    or die "failed to read $bytes bytes";
close $fh;
print $data;
[download]