Using LanX's suggestion in Re^3: Distinguish between missing and undefined arguments with subroutine signatures (semipredicate problem), I finally found a practicable solution to my problem. 

# 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
    }
}

[download]

Thanks, Rolf!

Greetings,
-jo

$gryYup$d0ylprbpriprrYpkJl2xyl~rzg??P~5lp2hyl0p$

In reply to Re: Distinguish between missing and undefined arguments with subroutine signatures by jo37
in thread Distinguish between missing and undefined arguments with subroutine signatures by jo37

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