The trick that CGI uses is all in the following sub: 
sub self_or_default {
    return @_ if defined($_[0]) && (!ref($_[0])) &&($_[0] eq 'CGI');
    unless (defined($_[0]) && 
            (ref($_[0]) eq 'CGI' || UNIVERSAL::isa($_[0],'CGI'))
                 # slightly optimized for common case
            ) {
        $Q = $CGI::DefaultClass->new unless defined($Q);
        unshift(@_,$Q);
    }
    return @_;
}

[download]
They preprocess the args to every function this way, and as long as the first arg is 'CGI' or an object that inherits from CGI they leave the arguments alone. Otherwise they prepend a global to the list.

This is nice but imposes quite a bit of overhead.

Another way of doing the same thing is to make the procedural interface be in a different package, and have a well thought-out AUTOLOAD sub that creates needed procedural wrappers at run-time. More complex but probably more efficient than what CGI does.

Another solution that I have used is to think functionally. There you actually have your error handler be an anon sub. By default it is your default, but it can be anything you want. In this case you would leave your package alone and in your CGI script say: 

$handler ||= \&Err::err;
# time passes
do_the_right_thing() || $handler->(title=>'Oops',msg=>'something went 
+wrong');

[download]
If $handler is left alone, well you get the default. But anyone who wants can change it.

I have often found that this functional approach is a more efficient way to implement optional hooks. YMMV.

In reply to RE (tilly) 1: OO vs. procedural style by tilly
in thread Handling cascading defaults by markjugg

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