Sara, you could paste sample of your code, that we could know where is problem.

You can do something like that:

use Getopt::Std;
getopts('u:x:m:', \%Opts)
$file = $Opts{'f'} or die "Where is your file?";
$x = $Opts{'x'};
$m = $Opts{'m'};
[download]

-f is required, -x and -m is optional,