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...

Ah yes, quite right. I missed that in my haste.

OP, the function you're calling doesn't appear to be standard DBI, so I can't advise you (the OP) on how to handle exceptions, but in general, it will probably be one of the following:

• The function might die() on error; use eval
• It may return an empty list, in which case your $mx_registered_process_id will be undef (but undef might also indicate something else, such as a NULL column, so be careful; assign the function return to an intermediary @array if that's the case).
• It may use a package scalar to hold error values (which is set to undef if there was no error.

Again, this is all just speculative, since I don't know the semantics of your db_select_row() function.