use strict;
use warnings;

use Inline C => <<'__EOI__';

SV* f(SV* sv, I32 pos, I32 len) {
    SV* targ = newSV_type(SVt_PVLV);
    sv_magic(targ, NULL, PERL_MAGIC_substr, NULL, 0);
    LvTYPE(targ)    = 'x';
    LvTARG(targ)    = SvREFCNT_inc_simple(sv);
    LvTARGOFF(targ) = pos;
    LvTARGLEN(targ) = len;
    return targ;
}

__EOI__

use Devel::Peek qw( Dump );

my $x = "abcde";
$_=uc for f($x,1,3);
print("$x\n");        # aBCDe
[download]

If not, just copy the two functions relating to substr in mg.c

Yep! Bin there, done that. Trouble is, the lvalueness doesn't survive assignment:

#! perl -slw
use strict;
use warnings;

use Inline C => <<'__EOI__';

SV* f(SV* sv, I32 pos, I32 len) {
    SV* targ = newSV_type(SVt_PVLV);
    sv_magic(targ, NULL, PERL_MAGIC_substr, NULL, 0);
    LvTYPE(targ)    = 'x';
    LvTARG(targ)    = SvREFCNT_inc_simple(sv);
    LvTARGOFF(targ) = pos;
    LvTARGLEN(targ) = len;
    return targ;
}

__EOI__

use Devel::Peek qw( Dump );

my $x = "abcde";
my $r = f($x,1,3);
Dump $r;
print $r;

__END__
C:\test>ike2-lv.pl
SV = PVNV(0x410d558) at 0x97890
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x40fa988 "bcd"\0
  CUR = 3
  LEN = 8
bcd
[download]

The effect I am after is persistant lvalueness as demonstrated here (and back up there somewhere):

C:\test>perl -MDevel::Peek=Dump -E"
$s = 'fred'; 
$r = \substr $s, 1, 2; 
$$r = 'oo'; 
say $s; 
Dump $_ for $s, $r; 
$$r = 'eu'; 
say $s;  
Dump $_ for $s, $r
"

food
SV = PV(0x16d6f0) at 0x97b60
  REFCNT = 3
  FLAGS = (POK,pPOK)
  PV = 0x16b3b8 "food"\0
  CUR = 4
  LEN = 8
SV = RV(0x97be8) at 0x97bd8
  REFCNT = 2
  FLAGS = (ROK)
  RV = 0x97c80
  SV = PVLV(0xe2c48) at 0x97c80
    REFCNT = 2
    FLAGS = (PADMY,GMG,SMG,pPOK)
    IV = 0
    NV = 0
    PV = 0x16b478 "oo"\0
    CUR = 2
    LEN = 8
    MAGIC = 0x103708
      MG_VIRTUAL = &PL_vtbl_substr
      MG_TYPE = PERL_MAGIC_substr(x)
    TYPE = x
    TARGOFF = 1
    TARGLEN = 2
    TARG = 0x97b60
    SV = PV(0x16d6f0) at 0x97b60
      REFCNT = 2
      FLAGS = (POK,pPOK)
      PV = 0x16b3b8 "food"\0
      CUR = 4
      LEN = 8
feud
SV = PV(0x16d6f0) at 0x97b60
  REFCNT = 3
  FLAGS = (POK,pPOK)
  PV = 0x16b3b8 "feud"\0
  CUR = 4
  LEN = 8
SV = RV(0x97be8) at 0x97bd8
  REFCNT = 2
  FLAGS = (ROK)
  RV = 0x97c80
  SV = PVLV(0xe2c48) at 0x97c80
    REFCNT = 2
    FLAGS = (PADMY,GMG,SMG,pPOK)
    IV = 0
    NV = 0
    PV = 0x16b478 "eu"\0
    CUR = 2
    LEN = 8
    MAGIC = 0x103708
      MG_VIRTUAL = &PL_vtbl_substr
      MG_TYPE = PERL_MAGIC_substr(x)
    TYPE = x
    TARGOFF = 1
    TARGLEN = 2
    TARG = 0x97b60
    SV = PV(0x16d6f0) at 0x97b60
      REFCNT = 2
      FLAGS = (POK,pPOK)
      PV = 0x16b3b8 "feud"\0
      CUR = 4
      LEN = 8
[download]

But I'd like to avoid the need for taking a reference and using indirection.

And to keep this all in one thread: No to "Are you really asking about making an lvalue function?"

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP PCW It is as I've been saying!(Audio until 20090817)

Magic isn't copied on assignment, not even for substr magic.

You did $r = func for my function, but $r = \func for substr. This difference in the code accounts for the difference in the results.

If I take your "persistant lvalueness" example and simply change substr to f, I get the same output as before the change.

Feel free to return a reference to targ instead of targ itself if that's what you want.