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
[download]

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.