s/href=([^"\s>]+)/href="$1"/g

##</code><code>##

<a href="http://www.google.com/">Google</a> uses .html files.

##</code><code>##

s/(href="[^"]*)\.html"/$1.asp"/g
s/(href="http://[^"]*)\.asp"/$1.html"/g

##</code><code>##

s{(href="[^"]*\.html")}{
    my $s= $1;
    $s =~ s#\.html"$#.asp"#
        unless  $s =~ m#^http:#;
    $s
}ge;