They are poison. You should only use them when you have to, like forcing an array or subroutine reference. Here's two typical examples:

sub my_grep(&) { ... }
sub my_pop(\@) { ... }
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Putting in scalars (i.e. ($$)) is asking for trouble since it converts any arrays to scalars automatically. I found this really quite worrying, and you can see my brief rant in Function Prototypes and Array vs. List (Pt. 2).

This all comes from a technique of jamming arrays into functions, which I do all the time to be efficient. Things like this:

my $sth = $dbh->prepare("SELECT id,name,age FROM foo");
$sth->execute();

while (my $row = $sth->fetchrow())
{
     $self->some_function(@$row);
}
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Now, if you've prototyped your function, you're going to get the number 3 every time. This is put in to the 'id' field, so you might actually think it's valid data, too.

Instead, just leave those things off. That's why merlyn says "...and this is why we tell people DO NOT USE PROTOTYPES".

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