Speed. I moved mostly to Ruby years ago, but when doing lots of text processing, a trivial script in Perl runs a lot faster than a similar trivial script in Ruby.

use strict. In Perl you can prevent something like the following from compiling. You can't with Ruby (unless they've finally addressed that).

   $foo = 2;
   if (some_test()) {
      $fooo = 3;
   }

Shouldn't that be:

    my $foo = 2;

?

I was leaving the declaration implicit.