I don't see anything in the substitution chapter that directly uses one of my favorite techniques -- calling subs on the rhs of a substitution. It may be obvious, if you think about it, but it's worth discussion. Everything (and Slash) handle links in textfields with something like this:

$text =~ s/\[([^\]]+)\]/parseLink($1)/eg;

sub parseLink {
    my $link = shift;
    my ($type, $title);
    ($type, $link, $title) = $link =~ m!(.+)://(.+)|(.+)!;
    $type ||= '';
    $title ||= $link;
    return makeLink($link, $title, $type);
}
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Somewhat hastily reconstructed out of memory. It's a little more optimized in the code, I believe. It's also a very nice way to handle more complex transformations.