Do not parse CGI input data by hand. There is a perfectly good module that does it for you. Example:

use CGI qw(:standard);

my $param1 = param('param1');
my $param2 = param('param2');

if(! defined($param1) ) { # Run the following code if $param1 is null
   die "param1 is null!\n";
}
if(! defined($param2) ) { # Run the following code if param2 is null
   die "param2 is null!\n";
}
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Update: Added :standard import for CGI.pm, as per merlyn's suggestion (oops).

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated