Try this:

  #!/usr/bin/perl

  use re "eval";

  # (?{ CODE }) is a classical zero-width assertion
  # that allways succeeds and it is used only for its
  # side-effects.

  my $non_zero_width = 'a(?{ print 1 })';  
  my $zero_width     = '(?{ print 2 })';

  # These two should be equivalent according to perlre.
  my $re1 = qq/ (?: $non_zero_width | $zero_width )* /;
  my $re2 = qq/ (?: $non_zero_width )* | (?: $zero_width )? /;

  # But are they really?

  $_ = 'aaabbb';

  print "\n-----------------\n";

     /$re1/x;

  print "\n-----------------\n";

     /$re2/x;

  print "\n-----------------\n";
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The output is:
-----------------
1112
-----------------
111
-----------------
which proves my point.