substr($s, $pos, $len) = $x;

##</code><code>##

substr($s, $pos, $len, $x);

##</code><code>##

int
Perl_magic_getsubstr(pTHX_ SV *sv, MAGIC *mg)
{
    STRLEN len;
    SV * const lsv = LvTARG(sv);
    const char * const tmps = SvPV_const(lsv,len);
    I32 offs = LvTARGOFF(sv);
    I32 rem = LvTARGLEN(sv);
    ...
}

int
Perl_magic_setsubstr(pTHX_ SV *sv, MAGIC *mg)
{
    dVAR;
    STRLEN len;
    const char * const tmps = SvPV_const(sv, len);
    SV * const lsv = LvTARG(sv);
    I32 lvoff = LvTARGOFF(sv);
    I32 lvlen = LvTARGLEN(sv);
    ...
}

##</code><code>##

sub xs_func1 :lvalue;
sub xs_func2 :lvalue;

##</code><code>##

use attributes ();
for (qw( xs_func1 xs_func2 )) {
    no strict 'refs';
    attributes->import(__PACKAGE__, \&{$_}, 'lvalue');
}

##</code><code>##

my $mmap = $class->new( ..., endianess => ENDIAN_NATIVE );

++( $mmap->uint8(0x1234) );