Using a colon ":" instead of the equals sign indicates that the
option value is optional. In this case, if no suitable value is 
supplied, string valued options get an empty string '' assigned, while 
numeric options are set to 0.

##</code><code>##

use strict;
use warnings;
use Getopt::Long;
my $foo;

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

print "foo: $foo\n" if defined($foo);

print "Unprocessed by Getopt::Long\n" if $ARGV[0];
foreach (@ARGV) {
  print "$_\n";
}