Ok, now that just confuses the fuck out me.

Obviously this is a convoluted use of my and so I shouldn't worry about it too much. But these results don't match the explanation you gave earlier. If the theory is that both $a and $b get assigned a lexical at the beginning of the sub, than $a should be undef every sub call.

$a=1;
$b=2;
sub x {
    my (0?$a:$b) = shift;
    print"a=$a\nb=$b\n";
    $a=shift
}
x(3,30);
x(4,40);
x(5,50);
__END__
#Output
a=
b=3
a=30
b=4
a=40
b=5
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