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

my $chunk_len = 65536;
my $delim_len = length $delim;

read $fh, $buffer, $chunk_len, length $buffer;

my $rel_pos = -1;

while (length $buffer) {
    $rel_pos = index $buffer, $delim, $rel_pos + 1;
    if($rel_pos > -1) {
        do_checks_on(substr $buffer, 0, $rel_pos - 1);
        $buffer = substr $buffer, $rel_pos + $delim_len;
    } else {
        $buffer = substr $buffer, -$delim_len;
    }
    read $fh, $buffer, $chunk_len - length $buffer, length $buffer;
}