sub new { 
	my ($self, %param) = @_;
	my ($dbh, $foo) = map { $param($_) || croak "need $_" } qw(dbh foo);
	# ... stuff that uses $dbh and $foo ...
}

##</code><code>##

	croak "need a foo" unless UNIVERSAL::isa($foo, 'Foo');
	
##</code><code>##

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;
};