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";
}