I think the logic you mean is: 
use strict;
use warnings;
use Getopt::Long;

my $foo = undef;

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

my $sn = $ARGV[0];

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

[download]

I used the length test because 0 is a potentially valid input but evaluates to false in logical context. This would also hold for a whitespace-only input, e.g.:

perl script.pl --foo='   '

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: Options with optional values and condititions by kennethk
in thread Options with optional values and condititions by Qukz

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