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Perl: the Markov chain saw

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In attempting to formulate another reply to this little game of golf, I've hit upon some rather peculiar behaviour in list expression evaluation order. Consider the following recursive code:
sub li { shift; @_ ? (scalar @_, &li) : (); } print li qw(a b c d e f g); # prints 654321
So far so good... now substitute $#_ + 1 for scalar @_:
sub li { shift; @_ ? ($#_ + 1, &li) : (); } print li qw(a b c d e f g); # prints 654321
Que bueno! Moving on, lets remove the "+1" ...
sub li { shift; @_ ? ($#_, &li) : (); } print li qw(a b c d e f g); # prints -1-1-1-1-1-1
What is this? Apparently, $#_ is now evaluated after the recursive call. To my eyes, this is a rather unexpected result, especially when you consider how the comma operator behaves in a scalar context, and my first two examples. This phenomenon affects globals as well:
use vars '$x'; sub li { $x = shift; @_ ? ($x, &li) : (); } print li qw(a b c d e f g); # prints gggggg
Update: remarkably, I've just determined that this behaviour is tied to the way in which the subroutine call is performed: if the calls above are switched from &li to li(@_), the order of evaluation goes back to normal. How wierd!
Update2: actually, the above only works for the $#_, but not the globals. Stranger and stranger...

In reply to !@#$ $#_ by MeowChow

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