What's wrong with Data::Validate? Or, for that matter, just using CGI to do the hard parsing work, and validating from there?

#!/bin/perl -T
use CGI;
use Data::Validate ':math';

my $q = CGI->new();
foreach my $p ( $q->param ) {
   my $val = $q->param($p);
   if ( defined is_alphanumeric($val) ) {
      $q->param(-name=>$p, -value=>is_alphanumeric($val)); #untaint!
   }
   else {
      warn "Parameter '$p' is not alphanumeric!"
   }
}
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Don't reinvent wheels when people have already done the work for you! :-)