Oh, that's excellent! I'd forgotten that @_'s elements are aliases for the actual scalar parameters. We get the referencing for free, without the caller having to use a backslash.

This does what I need: 

sub verify {
        my $ref = \pop;
        my $string = @_ ? shift : "<default>";
        $$ref = $string;    # Dummy action for testing
}

verify $a;
verify "hello", $b;

print "a=$a b=$b\n";        # Expect a=<default> b=hello

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In reply to Re^2: Optional parameter before reference (!proto) by Neutron Jack
in thread Optional parameter before reference by Neutron Jack

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