Because Perl is the "glue" or "duct tape," if you will, of the internet.
That was succinct observation in its day, but I hardly think it applies now. Things have changed a whole lot. That's not to say perl can't be used today, but its relative value has decreased. "Duct tape" is no longer the ambition to admire.
Perl is the most resilient language. Even if things have changed, it will be back. With a vengeance.
Perl may have once been the duct tape of the internet, but it's pretty solidly into its 'drying and peeling off' phase right now.
Because as a PHP programmer Perl will have areas of familiarity which may make the learning process easier. In return, you will gain a greater appreaciation for why the PHP developers made the decisions they did by seeing how it was done in a language they borrowed heavily from. As a bonus you'll learn to use a language that is ubiquitous in all sorts of sysadmin tasks, and have another tool that you can use when PHP isn't the right one for the job. Finally, knowing Perl will allow you to use the huge wealth of CPAN modules available to help you solve almost any task. At worst it will enable you to port them to your language of choice.