my $buffer;
my $delim = '1234';

my $delim_len = length $delim;
my $chunk_len = 65536 - $delim_len;
my $read_len = read $fh, $buffer, $delim_len;

my $pos = 0;

while ($read_len) {
    my $rel_pos = -1;
    print "delim at offset: ", $pos + $rel_pos
        while ($rel_pos = index $buffer, $delim, $rel_pos + 1) > -1;

    $buffer = substr $buffer, -$delim_len;
    $pos += $read_len;
    $read_len = read $fh, $buffer, $chunk_len, $delim_len;
}