use strict;
use warnings;

my $foo = undef;

GetOptions(
    'foo:s' => \$foo,
);

my $sn = $ARGV[0];

if ($foo and $foo ne undef) {
    print "foo option used with optional value: $foo \n";
} elsif ($foo) {
    print "foo option used with no optional value \n";
} else {
    print "foo option not used at all \n";
}