No-execute wont make a difference, since what is executed is /usr/bin/perl which then reads a file (script) from /tmp/.

Depends on if the script itself gets executed (in which case it would help) or on if the script is passed to perl (in which case it won't). 

$ ls -l /tmp/ikegami/foo
-rwx------ 1 ikegami pg1404028 30 2011-01-04 12:23 /tmp/ikegami/foo*

$ cat /tmp/ikegami/foo
#!/usr/bin/perl
print "bad\n";

$ /tmp/ikegami/foo
-bash: /tmp/ikegami/foo: /usr/bin/perl: bad interpreter: Permission de
+nied

$ /usr/bin/perl /tmp/ikegami/foo
bad

[download]

But its really simple to circumvent.

No, it's not. You seem to have forgotten the threat against you which you wish to defend. The attacker either relies on the web server knowing where to find perl or on him knowing where to find perl.

Isnt there some sort of "autoloading" directory

No. Besides, this would at least as easy to circumvent as replacing perl itself.

Update: Elaborated on first point.

In reply to Re^3: perl pre-execution hook by ikegami
in thread perl pre-execution hook by logix

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