You should use Taint mode in your CGI scripts and check all input for dangerous things before using it.
Suggestion: Change your shebang line to enable warnings and Taint mode, like this:
#!/usr/bin/perl -wT
And then check all your input with regexes (un-taint it) to be sure that there's nothing scary in there.

$firstname =~ /^([\w ]+)$/;
$firstname = $1;
if( length($firstname) < 1){
  $firstname = "no valid name provided";
}
# now $firstname contains only
#  numbers, letters, spaces and underscores.
[download]

The particular regex I used may not meet your needs, and sanitizing the other pieces may be more difficult, but this is the general idea I think you'll want.

Oh, to reply more to the question you actually asked - add this: "use CGI::Carp( 'fatalsToBrowser' );" to your script while you're debugging. Then you'll get a better description than "HTTP 500" when things go wrong.

Good luck!

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