Which Perl is that?

$ corelist File::Spec
File::Spec was first released with perl 5.00405
[download]

Perl 5.004_05 was released ~16 years ago.

And why can't you install modules yourself? A Guide to Installing Modules shows how to install modules locally without root access.

But anyway, if you really have been restricted that much, I'm sorry to hear that... LanX's regex solution will work as long as you don't need the script to work on lots of different OSes.