$ cat XREL.pm
package XREL;
use strict;
use base 'HTML::Scrubber';

sub _validate {
    my ($self, $t, $r, $a, $as) = @_;

    if ( $t eq 'a' ) {
        $$a{ rel } = 'nofollow';
        push @$as, 'rel' unless grep { /rel/ } @$as;
    }
    $self->SUPER::_validate( $t, $r, $a, $as );
}

1;