$ perl -wE'
use Devel::Peek;
use Encode;
use utf8;

my $str = "ä";
my $substr = substr($str, 0, 1);
Dump $substr;
$str = encode_utf8($str);
$substr = substr($str, 0, 1);
Dump $substr;
'
SV = PV(0xa0ff780) at 0xa11cab0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0xa1209b8 "\303\244"\0 [UTF8 "\x{e4}"]
  CUR = 2
  LEN = 4
SV = PV(0xa0ff780) at 0xa11cab0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0xa1209b8 "\303"\0
  CUR = 1
  LEN = 4