$ cat scrub.pl
#!/usr/bin/perl

use warnings;
use strict;
use XREL;

my $scrubber = XREL->new( allow => [ qw[ a p b i u hr br ] ] );
$scrubber->rules(
        a => {
            href => 1,
            rel => qr/^nofollow$/i,
            '*'  => 0,
        }
);

my $html = q[<a href="http://perlmonks.org">link </a>];
print $scrubber->scrub($html), $/;
$html = q[<a href="http://perlmonks.org" rel="nofollow">link </a>];
print $scrubber->scrub($html), $/;
$html = q[<a href="http://perlmonks.org" rel="xxx">link </a>];
print $scrubber->scrub($html), $/;
$html = q[<a href="http://perlmonks.org" rel="xnofollow">link </a>];
print $scrubber->scrub($html), $/;

__END__
output:
<a href="http://perlmonks.org" rel="nofollow">link </a>
<a href="http://perlmonks.org" rel="nofollow">link </a>
<a href="http://perlmonks.org" rel="nofollow">link </a>
<a href="http://perlmonks.org" rel="nofollow">link </a>