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;
};