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

    croak "need a foo" unless UNIVERSAL::isa($foo, 'Foo');

package Foo;
use Carp;
...
    croak "need number" unless $n =~ $RE{num}{real} 
    $self->{total} += $n;
};