with 5.10, you'd get [...] the low-byte values of the codepoints, with the high-byte part being truncated. With warnings enabled, you also get "Character in 'H' format wrapped in unpack at...".

It's odd that it doesn't warn or croak with "Wide character in ...".

If you want to dump the internal buffer, 

use Encode qw( _utf8_off );

sub internal {
   _utf8_off( my $s = shift );
   return $s;
}

my $s = "\x{1234}\x{5678}";   # string with utf8 flag on

print unpack("H*", internal($s)), "\n";

[download]

Update: Fixed error identified in reply.

In reply to Re^3: Understanding pack and unpack changes for binary data between 5.8 and 5.10 by ikegami
in thread Understanding pack and unpack changes for binary data between 5.8 and 5.10 by dkg

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