if( *a == 9 ) { ## Time to prepend another byte
        if( SvOOK( n ) ) { ## If we've some reserve left use it
            SvLEN_set( n, SvLEN( n ) +1 );
            SvCUR_set( n, ++l );
            SvPV_set( n, --a );
        }
        else { ## else insert 100 more bytes and use sv_chop 
               ## to reserve 99 of them for later
            char pad[100] = { 0, }; ## Initialise the reserve to all zeros
            sv_insert( n, 0, 0, pad, 100 );
            sv_chop( n, SvPVX( n ) + 99 );
            a = SvPVX( n );
            ++l;
        }
    }
    ...