I have to know where perl is to run eval...... and that defeates the purpose.

Yes and no. If you don't know, but the system does, this works. Which leads to:

1) The sysadmin should know where Perl is on your system.
2) If you're the sysadmin, then you know where Perl is on your system.

(1) can fail because the sysadmin doesn't know (or has some other problem I won't go into). (2) can fail for similar reasons -- only the perspective changes.