my $postdata = Encode::encode('UTF-8', "x=1&y=2&z=greektext"); # for example, 
my $p1 = "$nonsense".'&'.$postdata; # yes & needed
my $p1_utf8 = Encode::encode_utf8($p1);
my $api_sha256 = Digest::SHA::sha256($p1_utf8);
my $message = Encode::encode_utf8($api_path)
  . Encode::encode_utf8($api_method)
  . $api_sha256; #<< last one is binary

# ... and post after some more massaging