I was in such situation several times, and mostly ended up adding negation (or other modifier) to the parameter itself, then post-processing it with regexp or something. I.e.

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

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

if ($foo) {
    print $foo =~ /!(.*)/ ? "All but $1" : "Only $foo";
};
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bash$ perl all-but-foo.pl --foo 4
Only 4
bash$ perl all-but-foo.pl --foo \!4
All but 4
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