Good catch, but this works

  DB<106> (my ($ar) = () )     || print "bla"
bla
  DB<108> (my ($ar) = (1) )    || print "bla"
 => 1

  DB<109> (my ($ar) = (0) )    || print "bla"
 => 1

  DB<110> (my ($ar) = (undef) )|| print "bla"
 => 1

  DB<111> (my ($ar) = (undef,undef) )|| print "bla"
 => 2
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list-assignment in scalar context counts and the count is always defined. (see also goatse-operator for comparison.)

  DB<119> scalar ( my ($ar) = () )
 => 0
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so only empty-list is 0 and false.

But I'm not sure if I'm in favour of such constructs...