I was hoping to avoid copying i.e. re-use allocated string data. Thanks to your tip, I think a working solution can be:

void _replace( Imager img, SV* str ) {
    sv_force_normal( str );
    unsigned char* ptr = SvPV_nolen( str );
    myfree( img-> idata );
    img-> idata = ptr;
    SvPV_set( str, savepv( "" ) );
}
[download]

It works, though I'm still not sure, is it really OK? Or too ugly? Or scandalously incorrect and will bite me some time later?

:) flip fones are hard to use, i wanted to link perlclib/perlapi

Well, it all looks ok to me, sv_force_normal , SvPV_nolen/SvPVk myfree , SvPV_set

But it doesn't work for me with citrusperl v5.16.1 and gcc-4.6.3

A newer gcc like with latest strawberry might change that :)

Thank you. Yes, it works with Strawberry 5.20.2, gcc 4.8.3 (i686-w64-mingw32).