Well, I understand that I should explicitly state what do I consider as a threat and what not but that in itself is to difficult for me now. Besides when I am asked if this code can be trusted, usually the person who asks this has even less understanding than I do.

That's easy. In that case, the answer is No, I cannot trust the code. You just don't have enough information. Even if you do have perfect knowledge of the third party code, you still don't know enough of your own system.

Suppose the code you're looking at contains something like this:

use Fcntl ':flock';
my $lockfile = "/var/lock/frobnitz";
open $lh, "+<", $lockfile or die $!;
flock $lh, LOCK_EX or exit;
sleep 600;
exit;
[download]

Harmless? Probably. Unless you have another program running that uses /var/lock/frobnitz for locking - that one will be blocked for 10 minutes.