use warnings;
use strict;
use Getopt::Long;

my $foo;
GetOptions(
    "foo=s" => \$foo,
);

if ($foo) {
    print $foo =~ /!(.*)/ ? "All but $1" : "Only $foo";
};
##</code><code>##

bash$ perl all-but-foo.pl --foo 4
Only 4
bash$ perl all-but-foo.pl --foo \!4
All but 4