my $windowsize = 500;
use constant BLOCKLENGTH => 4096;
my $buffer = "";
my $offset = 0;
while(my $r = read FH, $buffer, BLOCKLENGTH) {
    my $i = -1;
    until(($i = index($buffer, "x", ++$i)) < 0) {
        printf "found 'x' at %d+%d\n", $offset, $i;
        if($i+$windowsize > length $buffer) {
            # get rid of what we no longer need, or we might end up with a buffer holding the whole huge file:
            $offset += $i;
            $buffer = substr $buffer, $i;
            $i = 0;
            # append a new buffer (assuming BLOCKLENGTH >= $windowsize):
            $r = read FH, $buffer, BLOCKLENGTH, length $buffer;
            last if $windowsize > length $buffer; #not long enough
        }
        # do something here...
        printf "offset for 'x' is %d, found '%s' at %d\n",
          $offset + $i, substr($buffer, $i + $windowsize - 1, 1),
          $offset+$i+$windowsize-1;
    }
} continue {
    $offset += length $buffer;
}