The way I read your question you're not talking about varying what goes into your HTML Template, you're talking about varying what goes into your script?

As this is clearly a CGI script, couldn't you just use a parameter and do

if{$q->param('mode' eq 'foo')}{
  # do some stuff
}
if{$q->param('mode' eq 'bar')}{
  # do some other stuff
}
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which would mean you just had one big script?

If you don't like that, why not

if($mode eq 'foo'){
        require 'foo.pl';
}
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which will leave you one small script which calls others as required?

You could even do this:

require "$mode.pl";
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But it's got "security hole" written all over it.