# Unique catcher for a missing argument
use constant MISSING => bless {}, __PACKAGE__ . '::__missing__';

# Helper sub, that either checks its argument against the catcher
# or otherwise returns the catcher itself.
sub missing {
    @_ ? $_[0] && $_[0] eq MISSING : MISSING;
}

# To be used with signatures in various places:
sub foo ($self, $val=missing) {
    if (missing $val) {
        # getter
    } else {
        # setter
    }
}