Re: Strange benchmark result that has me totally confounded

[bart]

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;

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

[jpfarmer]

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.

[chromatic]

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.

[simonm]

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