use Getopt::Long;

my $days = 30;
GetOptions( 'days=i', \$days );
print $days, "\n";

##</code><code>##

$ perl script.pl
30

$ perl script.pl -days 100
100

$ perl script.pl -days
Option days requires an argument
30

$ perl script.pl -days many
Value "many" invalid for option days (number expected)
30