But I suppose you don't want the sub missing() to be available as method the way ->foo() is?

That doesn't harm at all. Calling $obj->missing just returns false, which is correct :-)

However, your objection discovered a flaw in my approach. The missing sub needs to be prototyped to go one step further: multiple optional arguments. And then slurping starts hurting :-)

This leads to:

#!/usr/bin/perl

use v5.16;
use warnings FATAL => 'all';

package Foo;
use experimental 'signatures';
use constant MISSING => bless {}, __PACKAGE__ . '::__missing__';

sub missing :prototype(;$) {
    @_ ? $_[0] && $_[0] eq MISSING : MISSING;
}

sub new ($class) {
    bless {}, $class;
}

sub foo ($self, $foo=missing, $bar=missing) {
    say "foo is missing" if missing $foo;
    say "bar is missing" if missing $bar;
}

package main;

my $foo = Foo->new;
say "none:";
$foo->foo;
say "one:";
$foo->foo(1);
say "two:";
$foo->foo(1, 2);
print "\n";
say 'object foo is not missing' unless $foo->missing;

__DATA__
none:
foo is missing
bar is missing
one:
bar is missing
two:

object foo is not missing
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