For the most part the biggest hurdles about getting them into Perl are going to be the same as they are for anyone.
Here are the PHP centeric points that spring to mind:
Use strict or die.
PHP coders can get into a lot of bad habits. As always, this will save everyone a lot of pain.
CPAN is your friend.
The power and flexibility of the CPAN will come as an epiphany to PHP
programmers. PEAR is lightyears behind it.
The difference between == and eq.
This is always a big hurdle.