my $s = "YOUR_NEEDLE"; 
open my $f, "<", "YOUR_FILENAME" or die "open error: $!"; binmode $f or die "binmode error: $!"; 
my($b0, $b1) = ""; my($r, $g) = 0; 
do { $b1 = $b0; $r = read($f, $b0, 1024); defined($r) or die "read error: $!"; $r == 0 and die "string not found"; } until 
         0 <= ($g = index($b1 . $b0, $s)); 
my $pos = tell($f) - length($b0) - length($b1) + $g + length($s);