Ah, if the input string contains only characters in 00..7F, the returned string will use the UTF8=0 internal storage format.

$ perl -e'
   use Devel::Peek qw( Dumpe );
   my $s = "ab";
   utf8::upgrade($s);  # Force UTF8=1 storage format.
   Dump($s);
   my $ss = substr($s, 0, 1);
   Dump($ss);
'
SV = PV(0x7fffdcf3dca0) at 0x7fffdcf6ca78
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x7fffdcf5dbf0 "ab"\0 [UTF8 "ab"]
  CUR = 2
  LEN = 10
SV = PV(0x7fffdcf3dd30) at 0x7fffdcf6cb50
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x7fffdcf54050 "a"\0
  CUR = 1
  LEN = 10
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But like I said, it's Perl's perogative to pick whatever internal storage format it wants.