Firstly, take a look at the following:

• perlthrtut - Perl threads tutorial
• threads - Perl interpreter-based threads
• threads::shared - Perl extension for sharing data structures between threads

In addition, here's a couple of points based on the code you've supplied:

• You should generally be using subroutine() instead of &subroutine(), i.e. no leading ampersand (&). See perlsub.

  Update: See ++BrowserUk's comment below. threads::shared has:

  $var = &share([]);   # Same as $var = shared_clone([]);
  [download]

• $a (and $b, which you haven't used) are special variables used by sort and a number of other list-related functions (see List::Util and List::MoreUtils for some examples). $a and $b are described in perlvar.

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