It all depends on what 'mainstream' means.
Indeed, and I find any definition of 'mainstream' that doesn't include Perl to be full of crap.
For most of the 90's, CGI was practically synonymous with Perl. Sure, you could write CGI in C, C++, shell, or TCL, but almost nobody did. But hey, I'm sure the server side didn't count as 'mainstream'.
Okay, so you could argue that was the past and Perl's dead. I still disagree. The vast majority of Linux distros include Perl in their baseline installation. You can talk about the lack of Linux on the desktop but you'd be a fool to discard the massive uptake of Linux (and thus Perl) in the server space.
Perl comes as part OSX too. I suppose that means Apple isn't 'mainstream' either.