If you see this message, your code includes an undefined variable that you have used as if it was already defined and initialized. For example:

my $param = $q->param('test');
print $param;

You can fix this fairly painlessly by just specifying a default value:

my $param = $q->param('test') || '';
print $param;

In the second case, $param will always be defined, either with $q->param('test')'s return value or the default value the empty string ('' in our example).