can you please help me come up with some *objective* bullet points in defense of continuing to use Perl?
You go to perlmonks for objective bullet points? Right...
If you cannot come with reasons to use Perl, I think you've already lost your case. It's your manager, your department, your website. Explain to him why you used Perl. After all, your reason are likely to be different from mine.
I mostly use subjective reasons to determine which tool to use for which task. Primary reason to pick X over Y is, "I know X, I don't know Y". I don't think there's a more subjective reason. Neither do I think it's wrong to have subjective reasons.