txpsj has asked that I explain how this works, so here goes.

All packages implicitly inherit from the UNIVERSAL package. This is like every package's @ISA always ending in 'UNIVERSAL' no matter what you set a package's @ISA to be.

So if you do Data::Dumper->new(), then Perl looks for "sub Data::Dumper::new". If you've already done use Data::Dumper or require Data::Dumper, then Perl finds it and things progress as normal.

But, if Perl doesn't find it, it looks in @Data::Dumper::ISA for packages that Data::Dumper inherits from in case the new() method is supposed to be inherited. But no Data::Dumper::new() means no @Data::Dumper::ISA yet so it is empty. But Perl still implicitly includes the UNIVERSAL package in the list and so looks for "sub UNIVERSAL::new". Well, we've given Perl one to find.

So Data::Dumper->new( "x", "y" ) becomes UNIVERSAL::new( "Data::Dumper", "x", "y" ). So we pull the package name off the argument list and do a require on it. This should define a real "sub Data::Dumper::new" which we then call.

All this brings to my mind some improvements I should make. First, it would be nice to "goto" the real new() so that the call stack doesn't show the extra UNIVERSAL::new() in case new() wants to look at caller(), for example. Also, if require Data::Dumper didn't define "sub Data::Dumper::new", then we'd be in a infinite loop.

So here is a better version (updated):

package UNIVERSAL;

use strict;
sub new {
    my( $pkg )= @_;
    eval "require $pkg; 1"
        or  die "$@";
    my $meth= $pkg->can( "new" );
    die "Package $pkg defines no new() method"
        if  $meth == \&UNIVERSAL::new;
    goto &$meth;
}

1;
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#!/usr/bin/perl -w -mAutoNew
use strict;
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Going down this path is dangerous. But since you are going down it, may I suggest the additional hack to use CPAN.pm to fetch the specified module if the require fails? :-)

In this node RE: Factory Pattern for Class Heirarchy, I took this concept to an extreme and wrote up a UNIVERAL::AUTOLOAD method that will load the package when you refer to ANY method in the package. It's a horrible abuse of the UNIVERSAL and AUTOLOAD methods but it was education to learn how they worked and interacted.