You might want to take a look at threads like How to answer "Perl is not secure" objections?, Tired of "Perl is dead" FUD ?, Perceptions of Perl - views from the edge, "Tired of FUD" Followup: Where the Jobs Are, and What can bring the excitement back to Perl?. For references off of Perlmonks, maybe look at Tim Bunce's Perl Myths, or the TIOBE language index, which shows Perl as one of the top ten most popular languages right along with PHP.

Of course, if you're the lone Perl person in a sea of PHP programmers, then your company's best interests might be to make sure everyone understands the code. The cheaper way to do that in the short term is to tie one person up changing the code rather than tying up almost all of the programmers with the task of learning Perl. Everyone else in your company learning Perl might be preferable, but what we prefer and what's prudent aren't always the same.