This is a single detail, not a full answer to your question, but for me, the mark of someone who's truly become oriented to Perl is someone who replaces this idiom:
for (my $i = 0; $i < @a; $i++)
{
&do_something_with($array[$i]);
}
with this idiom:
foreach my $a (@array)
{
&do_something_with($a);
}
Teach them lists as lists, not just as arrays. Teach the awesome data structures. Teach lists, hashes, lists of hashes, hashes of lists, hashes of hashes. That's where the biggest step up in power comes in over most conventional strongly-typed languages. (Ironically in the Java shop I joined this year, we sling around tons of arbitarily deep nested HashMap structures. Too many, in fact; I'm the one arguing for doing a bit less of it! But in Perl, this sort of thing is vital.)
