use Benchmark qw(:hireswallclock cmpthese);
my $bappo = 'bappo';
my $foo = "foo bar baz bippo $bappo zappo";

cmpthese( -3, {
  extended => sub {
    $foo =~ /
      $bappo
      \s+
      (zappo)
    /x;
  },
  comment => sub {
    $foo =~ /
      $bappo     # this is a comment
      \s+        # inside an extended
      (zappo)    # regular expression
    /x;
  },
  no_comment => sub { $foo =~ /$bappo\s+(zappo)/ },
} );
__END__
               Rate    comment   extended no_comment
comment    480920/s         --       -11%       -22%
extended   542294/s        13%         --       -12%
no_comment 614847/s        28%        13%         --