I've come to similar solutions as footpad with slight variations.

In the first case a required module isn't installed and the script dies with a link to the dist on CPAN:

eval("use XML::Simple 'XMLin'"); # required xml parser
if($@){&install_xml_simple(); exit}

sub install_xml_simple
{
print header; print qq~Install 
<a href='http://search.cpan.org/search?dist=XML-Simple'>XML::Simple</a
+>~; 
}
[download]

Second case just causes some form buttons and text to not be printed later:

my$trade = 0;
eval("use Compress::Zlib"); 
unless($@){$trade = 1} # if installed enable export/import
[download]

And another switch if the script doesn't find a self-created data file:

use vars qw($xpdat1 $dat1); 
my($xpdata,$data,$md,$nodat) = 0; # vars for next block
my$df = 'some.data';

if(-e $df){ 
     if(eval "require '$df'"){
          $md = (stat($df))[9];
          $xpdata = $xpdat1;
          $data = $dat1;
          }
     }
else{ $nodat = 1 }
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