in reply to Strange benchmark result that has me totally confounded

However, I'm still confused as to why there is such a dramatic change.
That's easy. You change your original items in @messages, so after one replacement, there's nothing left to replace.

Add 

    my @messages = @messages;

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at the top of each routine, and the difference becomes much less... Roughly around 22% of difference, just for the 2 extra substitutions.

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Re: Re: Strange benchmark result that has me totally confounded
by jpfarmer (Pilgrim) on Apr 16, 2004 at 22:45 UTC

    But I'm not modifying @messages, am I?

    Unless I'm missing something, I'm only modifying $post. The value of @messages isn't touched during the foreach loop because data from the array is copied into the scalar.

    That said, I see a change in performance as well when I use local to create a copy of @messages, and I just don't get it.

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      But I'm not modifying @messages, am I?

      Surprise! You are, due to the magic of variable aliasing: 

      my @messages = <DATA>;
foreach my $post ( @messages ) {
  $post =~ s/ /-/g;
}
print @messages;
__DATA__
one
foo bar
three words here

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[d/l]

      As perldoc perlsyn says:

      In other words, the "foreach" loop index variable is an implicit alias for each item in the list that you're looping over.

      I don't think local will do it either; not unless you want an empty array or are doing something a lot more clever than I can imagine. You need to reset the values of the array for each benchmark iteration.

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[d/l]

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