Re^2: How to Use Pack to Convert UTF-16 Surrogate Pairs to UTF-8? (updated)

I still think it's likely the OP is taking the wrong approach by trying to hand-roll a JSON decoder, so I did want to point out a few issues with your code - hopefully thereby also pointing out some of the pitfalls of hand-rolled approaches.

High surrogates range from U+D800 to U+DBFF, which your regex doesn't cover (e.g. unpack("H*", encode("UTF-16BE", "\N{VARIATION SELECTOR-256}")) eq "db40ddef").
Your regexes should probably also handle uppercase hex digits.
You might want to pass Encode::FB_CROAK to decode.
You don't need to loop over the strings with a regex and then a second regex, that's fairly inefficient; it can all be done in one regex.

Update: *sigh* Just because it can be done in one regex, still doesn't mean it should! But just for fun, enough rope to shoot oneself in the foot...

use warnings;
use strict;
use open qw/:std :encoding(UTF-8)/;
use Test::More tests=>1;
use Data::Dump qw/pp/;
use Encode qw/decode/;

my $str = "Ren\\u00e9 \\ud83D\\uDe06\\uDb40\\udDeF Fran\\u00E7oise"
    ." \\\\u2660\\U1234";

$str =~
s{  (?| (?&U)(d[89ab][0-9a-f]{2}) (?&U)(d[c-f][0-9a-f]{2})
      | (?&U)([0-9a-f]{4}) )
    (?(DEFINE) (?<U> (?-i) (?<!\\)(?:\\\\)*\\u ) )
}{ decode("UTF-16BE", pack("n*", hex $1, defined $2 ? hex $2 : ()),
    Encode::FB_CROAK) }iexg;

is $str, "Ren\xE9 \x{1F606}\x{E01EF} Fran\xE7oise \\\\u2660\\U1234",
    $str or diag pp $str;
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