$ perl -wE'
use Devel::Peek;
use utf8;
use Encode;
my $str = "ä";
my $substr = substr($str, 0, 1);
Dump $substr;

no utf8;

$substr = substr($str, 0, 1);
Dump $substr;
'
SV = PV(0x8af9780) at 0x8b16aa0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x8b1a9a8 "\303\244"\0 [UTF8 "\x{e4}"]
  CUR = 2
  LEN = 4
SV = PV(0x8af9780) at 0x8b16aa0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x8b1a9a8 "\303\244"\0 [UTF8 "\x{e4}"]
  CUR = 2
  LEN = 4