Replace recognizable pieces of code with stuff that does the same thing. Try to remove constants and temporary variables as much as possible, since they'll jump out at the viewer as something recognizable among all of the "junk". For instance, instead of

$var =~ s/[a-z]//g; print $var;
[download]

you could do:

eval qq|@{[print=>q<grep!y,\x61-\x7a,,,split''>]}|
[download]

Sure its longer, but how many people can guess what it will do by first glance? :)