Use Javascript and it'll push up the average page sizes (and page rendering times), use Perl and there will probably be more requests to the server (as bad data is submited and the page has to be resent)...
What I would do is have a combindation (dependent on what exactly is needed). Have Javascript form validation (on the <form onSubmit=> setting - please don't set the 'input type=submit' as 'input type=button' as this breaks older browsers), and then if it passes the validation, send it back to the server. This way, if they enter garbage in a field (such as letters in a number field etc), you can say 'Bad entry' before the data has to go back to the server and then the entire page resent.
If you can provide a few more details on what exactly you are trying to achieve, I'll probably be able to provide a few more specific answers.

In reply to Re: speed : client or server side by beebware
in thread speed : client or server side by silent11

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