$ perl -wE'
use Devel::Peek;
my $str = "ä";

use utf8;

my $sub = substr($str, 0, 1);
say length $sub;
Dump $sub;'
1
SV = PV(0x84556c8) at 0x8472aa8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x8476970 "\303"\0
  CUR = 1
  LEN = 4