use HTML::Entities;

my $input = "abc < ä > <p> Ö & ü xyz ";  # ISO-Latin-1

my $encoded;

$encoded = encode_entities($input, "\xA0-\x{FFFD}");
print "$encoded\n";

# now upgrade $input to character string (utf8)
# (by appending some unicode characters)
$input .= "\x{5555} \x{8888}";

$encoded = encode_entities($input, "\xA0-\x{FFFD}");
print "$encoded\n";

##</code><code>##

abc < &auml; > <p> &Ouml; & &uuml; xyz 
abc < &auml; > <p> &Ouml; & &uuml; xyz &#x5555; &#x8888;

##</code><code>##

s/([\xA0-\x{FFFD}])/$char2entity{$1} || num_entity($1)/ge;

##</code><code>##

$encoded = encode_entities($input, "^\x20-\x7E");  # do not encode printable ASCII chars