I tend to use Carp::Assert for this type of stuff.

use Carp::Assert;
sub foo {
  is(scalar @_, 2) if DEBUG;
}
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That way when your code goes into prodution you can have Perl optimize away the assertions by changing the use to a no

no Carp::Assert;
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I definately dislike it when code uses prototypes for doing this type of thing. The trade-offs are unacceptable.

Yves / DeMerphq
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Writing a good benchmark isnt as easy as it might look.