my $needed_param = 'need_default';
my $debug = 0; # default value numeric and not ""

# getting params with getopt with a strict syntax
# id est: if GetOptions fail may be better the program does NOT run at all
&help()if( ! GetOptions( 
                         'needed=s' => \$needed_param,
                         # [..]
                         'd=i' => \$_debug 
                       ) or ! $needed_param );

#then adjust values to a default or a maximum as needed

$debug > 3 ? $debug = 3 : $debug ;

sub help { "$0 -n [-d [0-3]] ... "}