How would I code "parameter asserts"? I checked the camel book, but there doesn't seem to be an "assert" keyword.

Depending on mood I may do something like this:

sub new { 
    my ($self, %param) = @_;
    my ($dbh, $foo) = map { $param($_) || croak "need $_" } qw(dbh foo
+);
    # ... stuff that uses $dbh and $foo ...
}
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or use a module like Params::Validate or Carp::Assert.

Do you personally use them in your code? Why or why not?

Yes, I tend to use them. For two reasons:

1. They make design decisions explicit in the code. Saying:

       croak "need a foo" unless UNIVERSAL::isa($foo, 'Foo');
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   says a lot about your code's contracts and, unlike paper documents or UML diagrams, won't get out of sync with the code.

2. It makes debugging easier. You get a specific error message near the place where the "bad" data went in (as opposed to a possibly less specific error message where the "bad" data gets used).

Reasons not to use them:

Efficiency. Assertions take time and in a tight loop or in a method that is called many times this can add up. You can use constants to allow you to switch your assertions on/off at compile time (the documentation for Carp::Assert has some examples of this.) There are some indications that there will be "real" assertions in the next perl5 that can be switched on/off without a compile time penalty.

When it's obvious that they're not needed (for some definition of "obvious" :-). For example, in:

package Foo;
use Carp;
use Regexp::Common; 

# ... rest of class ...

sub add {
    my ($self, $n) = @_;
    croak "need Foo object" unless UNIVERSAL::isa($self, 'Foo');
    croak "need number" unless $n =~ $RE{num}{real} 
    $self->{total} += $n;
};
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the two assertions are redundant (IMHO). If $self isn't a Foo object then somebody is calling my method as a subroutine and deserves all that they get. If $n isn't a number then the addition is going to throw a warning anyway.