Thanks. That's interesting, but not the case for my code which greatly simplified looks like this:

#! perl -slw
use strict;
package O;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'xso', CLEAN_AFTER_BUILD => 0;

typedef struct {
    SV**sv;
} O;

void *mem( size_t size ) {
    void *p;
    Newx( p, size, char );
    return p;
}

SV *new( char *package ) {
    O *o = (O*)mem( sizeof( O ) );

    SV *oh = newSVpv( (char*)&o, sizeof( O* ) );
    SV *rv = newRV( oh );

    o->sv = mem( sizeof( SV* ) );

    sv_bless( rv, gv_stashpv( package, 0 ) );
    SvREADONLY_on( oh );
    SvREADONLY_on( rv );
    printf( "N:rv:%p oh:%p o:%p\n", rv, oh, o );
    return rv;
}

int set( SV *rv, SV *in ) {
    O *o = *(O**)SvPV( SvRV( rv ), PL_na );
    printf( "S:rv;%p o:%p\n", rv, o );
    o->sv = newSVsv( in );
    return 1;
}

SV *get( SV *rv ) {
    O *o = *(O**)SvPV( SvRV( rv ), PL_na );
    printf( "G:rv;%p o:%p\n", rv, o );
    return newSVsv( o->sv );
}

void DESTROY( SV *rv ) {
    printf( "DESTROY:%s\n", SvPV_nolen( rv ) );
}

void CLONE( SV *rv ) {
    printf( "CLONE:%s\n", SvPV_nolen( rv ) );
}


END_C
package main;
use threads;
use Devel::Peek;

my $o = O->new();
print $o;

$o->set( "abcde" );
print $o->get();

print "\nthreaded\n";

async {
    $o->set( "12345" );
}->join;

<>;
print $o->get();

__END__
C:\test>xso
N:rv:000000000002E218 oh:000000000002E128 o:00000000040FCA98
O=SCALAR(0x2e128)
S:rv;000000000002E248 o:00000000040FCA98
G:rv;000000000002E248 o:00000000040FCA98
abcde

threaded

CLONE:O
S:rv;000000000435D8F0 o:00000000040FCA98
DESTROY:O=SCALAR(0x435d908)

G:rv;000000000002E248 o:00000000040FCA98
semi-panic: attempt to dup freed string at C:\test\xso.pl line 72, <> 
+line 1.
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