UPDATE: The OP was changed in a major way after my original reply. This no longer applies...

It seems like you just want to ignore your datestamp lines:

use strict;
use warnings;

while (<DATA>) {
    next if /\[datestamp\[/;
    print if /crap/;
}
__DATA__
[datestamp] [DEBUG] something here
[datestamp] [DEBUG] something else here
[datestamp] [ERROR] blah blah
<chunk of crap>
[datestamp] [INFO] bla bla bla
[download]