Perl && PHP website rewrite methodology help

[neoanomally]

Lay of the Land

I have been tasked with rewriting a website, (I'm new to the ways of PHP). This website is a way for users to transfer files, usually between 10 MB - 1 GB using HTTPS; which includes a coupling to a broad single-sign on service. Half the files are in PHP the other half use mod_perl and CGI (PERL was the primary part of the site during its inception in 99). The site has been untouched since 2008, the original developer left. Towards the end he did primarily everything in PHP but kept building on the perl since he didn't have a chance to rewrite it all before he left. I have little documentation that he left, but one of the few pieces I had was a very general/braod to-do list. And it looked like he was transferring most the PERL-cgi stuff to strictly PHP... Also there is no database with this program, it usees an XML log file. Any files on the server over X days are deleted.

Problem

My problem is there are over 100 perl files. Everything I have done in the past has been ground up or just added functionality to a site. The minor things that I have changed thus far have all been javascript and php. But the meat of the code is in perl and I'm not sure if I should read through and catalogue the code which would include learning perl... Or should I write the everything in PHP. There are many different directions I could go. I am competent in PHP, python, c++, java, but not PERL x.x

Questions
1. IS there anything that PHP can't do that the perl stuff is doing?
2. How would you suggest I approach this problem from a methodology standpoint. -What approaches can I take to tackle this problem?
3. Any reference material that I should use?

1. Depends what perl is actually doing for you. To me Perl is more 'powerful', but PHP seems far easier.
2. If you need to change Perl to PHP, I would have thought learning Perl would be essential.
3. http://www.cs.wcupa.edu/~rkline/perl2php/ for a guide to similar functions

//EDIT
Here's a tut I saw a while back. Found it very useful:

http://www.perl.com/pub/2008/04/23/a-beginners-introduction-to-perl-510.html

[neoanomally]

Well I understand high level stuff of what some of the stuff does. I think part of the problem with the site isn't the fact that the perl isn't powerful, it's more that I feel that the person didn't build the site to scale so there are a few performance bottlenecks. I wasn't sure if I should modify the perl and overall structure on how things are stored and retrieved. Redo it in PHP etc...

Part of it is I'm not sure how to approach and tackle this site since I'm used to doing everything ground up and designing first instead of taking something that's already made..

Thanks fort he input so far! Looks like I"ll be learning perl, I'll probably just try to figure out how to redesign the site so that it can handle the number of users we get. Thanks for the tutorial.