my $html = "<b>foo & bar</b>";
my %Map = ( '&' => '&amp;', '"' => '&quot;', '<' => '&lt;',
 '>' => '&gt;', "'" => '&#39;' );
my $RE = join '|', keys %Map;

# This of course encodes everything.
#$html=~s!($RE)!$Map{$1}!g; 

# Returns <b&gt;foo &amp; bar&lt;/b&gt;
$html=~s/(?:(?!<\w[^>]*)($RE))/$Map{$1}/g; 

print $html;