You could try something like this, i.e. put your not-to-be-replaced entites into a hash for easy lookup:

#!/usr/bin/perl

my %entities = map { $_ => 1 } qw(& " < >   ©
+);

while (my $line = <DATA>) {
    $line =~ s/(&(\w+?;)?)/exists $entities{$1} ? $1 : "&amp;$2"/eg;
    print $line;
}

__DATA__
foo &amp; &quot; bar &blah; &foo baz & ...
TEST&TEST;A&E&an &nbsp; HTML--- &nbsp; string - &lt; &copy; TVS&gt;
[download]

Would output:

foo & " bar &blah; &foo baz & ...
TEST&TEST;A&E&an   HTML---   string - < &copy
+; TVS>
[download]

(note that if you run this under strictures, it'll complain "Use of uninitialized value in concatenation" in case $2 is empty... I'll leave this as an exercise for you to fix :)

Update: I hadn't followed your other thread... so looking at wfsp's solution there, I'd say just use that instead and be happy :)