Despite Perl is powerfull and eficient, it has become the Cobol of the WWW

I'd agree with him.

I'd consider it a compliment.

The application needs to be written in a language with a future

Depends on the estimated lifetime of the application. Perl 5 isn't going to disappear in the next few years. Well before it does we'll have much more information on whether PHP N>5, Ruby 2, Perl 6 or some other interesting language is going to get the most market share and/or be most useful.

(Personally I think Perl 6 vs Ruby 2 is going to be interesting.)

But what whould you say to this person?

If it were me I'd be advising him to pick whichever language gives him the most bang for his buck now. The way that PHP is evolving it's not going to be any easier to maintain than Perl 5 in a few years time - so I don't think he'll save anything long term. I just don't think we have enough information to make a good guess on what (if any) "dynamic" language is going to be dominant in five years time.

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Update: Spooling mistook fixed. Thanks demerphq