But a note on naming: that rather looks like a "factory class", not like "converting to a subclass". If the class denominated by $self->{extension} has DUM::DUMMY in its @ISA, it is a subclass of DUM::DUMMY. Does the following (add sanity checks for @_ and such) fit into your design?

package DUM::DUMMY;
use base 'DUM::Base';

sub new {
    my $class = shift;
    my %args  = @_;
    my $ext;
    if ($ext = delete $args{extension} ) {
        my $backend = 'DUM::DUMMY::'.$self->{extension};
        eval "require $backend";
        if ($@) {
            # handle errors: die? return undef? set $@ ?
        }
    }
    my $self  = $class->SUPER::new(%args);
    bless $self, 'DUM::DUMMY::'.$ext if $ext;
    $self;
}
[download]

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}