"Best" is, of course, a matter of taste. Rather than "best", I'll answer this as "what editor should I use for Perl if I don't already have a strong preference for an editor?" I went on a short quest to answer this question for my students.
What I recommend now is Atom. It's free, open source, available on most platforms, well maintained, well documented, easy to use, and has a rich ecosystem of plugins. It works well enough for just about any language so you don't get trapped in a language-specific IDE. And it's powerful enough without being bloated.
I also recommend you learn the basics of vi. This is the editor available on any Unix machine, and you'll need to use it when you inevitably find yourself needing to edit files on a Unix machine. It is very powerful, but also very baffling.
Atom has rendered the rest of this answer obsolete.
Padre is an IDE dedicated to Perl, however it hasn't seen a release in years.
Emacs (and all its variants) and vim (and all its variants) remain excellent, powerful, but quite baffling to anyone not used to them. Still, you should know at least the vim
basics for when you inevitably find yourself needing to edit files on a Unix machine.
For Mac, there's TextMate, Aquamacs (emacs that acts like an OS X app with expected OS X hotkeys and menus) and TextWrangler.
On Windows Notepad++, Sublime Text and E Text Editor (no longer maintained) are good choices.
As for Perl 6, Perl 5 and Perl 6 are different languages with their own lives and development cycles. Neither one will kill the other.