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
But like I said, it's Perl's perogative to pick whatever internal storage format it wants.