Re: code in the update:

This part is almost certainly wrong:

my $postdata = Encode::encode('UTF-8', "x=1&y=2&z=greektext"); # for e
+xample, 
my $p1 = "$nonsense".'&'.$postdata; # yes & needed
my $p1_utf8 = Encode::encode_utf8($p1);
[download]

You're encoding your text twice.