I found a noticeable performance difference between the two - I think Perl is faster for simple scripts, but PHP probably performs better for more complex scripts that require library calls.
The analysis was done with a SOAP interface, and PHP consistently performed better. An equivalent script was written in both Perl and PHP.
I suspect this is due to the time required to interpret the Perl SOAP code.
It wasn't an exhaustive test, but if performance is a consideration, it's worth looking into.