Read Coping with Scoping by Dominus for the skinny on my vs local. The node I need a simple explantion of the difference between my() and local() is a long discussion on the topic, as well.

Myself, I seldom use the form of my with parens -- mostly because I create my variables when I need them. If I need a whole bunch of them at once, it's probably because I want the list context, such as at the top of subroutines:

sub foo {
  my ($bar, $baz, $troz) = @_;
  ...
}
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perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'