use strict;
use warnings;
## set up data in memory
my $tm=time;
my $file= "FRED.DAT";
my $data;
{
    open my $fh, '<', $file or die;
    local $/ = undef;
    $data = <$fh>;
    close $fh;
}

## report load statistics
my $str="XYZ";
my $lx=length($data);
my $tmx=time-$tm;
my $r=$lx/$tmx;
print "File $file cached $lx bytes in $tmx seconds @ $r bs\n";

## scan mega string for patterns and do stuff
my $nextposn=0;
my $offset=0;
## experiment with offset beyond 1949803025
## $offset=2500000000;
my $found=0;  my $occ=0;
while ($nextposn < $lx ) {
   $nextposn = index($data,$str, $offset);
   if($nextposn < 0) {goto NOMORE;}
   $found++;
   ## do stuff you need to do with the next characters
   ##
   ##
   $offset = $nextposn+1;
   ## report progress
   $occ++;
   if ($occ == 1000000) {print "$found so far $nextposn\n"; $occ=0;}
   }
## diagnostics  
NOMORE: 
print "Processed $found patterns, maximum position was $nextposn\n";