#!/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;

##</code><code>##

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