$ perl
use warnings;
use strict;

my @codes = ("<a ", "<a href");
my $codes_regex = join "|", map quotemeta $_,
# sort { length $b <=> length $a }
                                   @codes;
my $text = "testing a link: <A HREF=\"fooble.html\">boofle</a>";
print "in:  $text\n";
$text =~ s/($codes_regex)/lc $1/gie;
print "out: $text\n";
__END__

output with the sort:
in:  testing a link: <A HREF="fooble.html">boofle</a>
out: testing a link: <a href="fooble.html">boofle</a>

and without:
in:  testing a link: <A HREF="fooble.html">boofle</a>
out: testing a link: <a HREF="fooble.html">boofle</a>