Yep, it's fairly easy -- depending upon the tools you choose.

Suppose you have an online catalog and you have some way to configure what will appear on page 10 (which is left as an exercise to the coder). Maybe there's a sub generate_page() that takes in a number and just does the right thing. So, assuming you're using CGI or CGI::Simple, you'd just extract the number of the requested page via:

my $page_number = $q -> param('p');
if ($page_number =~ /^\d+$/) {
  generate_page($page_number);
} else {
  generate_error_page('One of us is confused.');
}
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The second question, well... it's not clear to me what you mean by "variables I declare elsewhere" so I can't speak to that. I'll try again, though, if you'll clarify that.