Consider:
shmem [qurx] ~ > perl -CO
use Encode qw/ _utf8_on /;
use Devel::Peek;
$s = " ";
_utf8_on( $s );
substr $s, 0, 1, "\x{b5}";
Dump $s;
print length $s, $/;
print "\$s: '$s'\n";
__END__
SV = PVMG(0xf64ff0) at 0xedd8c8
REFCNT = 1
FLAGS = (SMG,POK,pPOK,UTF8)
IV = 0
NV = 0
PV = 0xee2620 "\302\265"\0 [UTF8 "\x{b5}"]
CUR = 2
LEN = 10
MAGIC = 0xf66010
MG_VIRTUAL = &PL_vtbl_utf8
MG_TYPE = PERL_MAGIC_utf8(w)
MG_LEN = -1
1
$s: 'µ'
So this has nothing to do with IO layers; sysread apparently does substr, which just does the right thing. Magic.
"\x{b5}" is an iso-8859-1 (a.k.a) latin1 char and a valid UTF8 codepoint, whose UTF-8 hex value is 0xC2B5 (0302 0265 as octal) - MICRO SIGN (µ).
print chr hex 'b5' eq "\x{00b5}";
__END__
1
Your file handle was set to raw, so bytes are read. Since perl places the byte into an UTF-8 string slot, it converts it from the internal representation into UTF8 and happily places its char hex value (2 bytes) into the PV slot, to satisfy the utf8-ness.
Similar to what happens here (reversed):
use Encode qw( from_to _utf8_on );
use Devel::Peek;
$s = "\x{b5}";
Dump $s;
from_to($s, 'latin1', 'utf8');
_utf8_on $s;
Dump $s;
__END__
SV = PV(0x234fa90) at 0x2375870
REFCNT = 1
FLAGS = (POK,IsCOW,pPOK)
PV = 0x2372400 "\265"\0
CUR = 1
LEN = 10
COW_REFCNT = 2
SV = PV(0x234fa90) at 0x2375870
REFCNT = 1
FLAGS = (POK,pPOK,UTF8)
PV = 0x249b2c0 "\302\265"\0 [UTF8 "\x{b5}"]
CUR = 2
LEN = 10
Except that the 'magic' bits are missing (since none involved).
perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'
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