But what whould you say to this person?

Well, I would tell him that he's hiring me for the expertise, and that when it comes to technical issues, my expertise surpasses his, and that according to my judgement, Perl certainly isn't worse than PHP. And that if Perl becomes the "Cobol of the WWW", I'd consider that an honour, and that I doubt PHP would ever reach the status of it being compared with Cobol. I'd also say I'd write it in PHP, but I'll need time to get myself familiarized with the language, so the project will take longer then when I'd do it in Perl. And I doubt I'll like PHP, so when it comes to maintaining the application, it'll rank lower on my todo list than non-PHP tasks. (I won't tell him that it'll probably rank higher than the Java jobs which somehow always have my lowest priority).

But that's just me. I'm senior enough and around long enough that I can say that to most of the managers I've had the last decade or so.

Don't take it as a blanket recommendation to tell it to your manager.

Of course, a manager can have non- technical issues to favour PHP over Perl. And he will have the last say in it. But I'm hired for my technical expertise - and I will voice my opinions and objections. That's part of my job. (I don't think singing I told you so is part of the job, but I've done so in the past).