Many thanks (again) for this code, it is very generous of you and so much easier to learn from than the pure reference material of the 'guts and 'api docs.

I set about adapting the above sub to my needs which are slightly less demanding that yours. The naming of sv_chop is (historical; set in stone; not of your making), slightly confusing with respect to Perl's chop as they operate on different ends of the string. That makes sv_unchop even more confusing :)

Anyway, whilst reading the docs on sv_chop, I noticed sv_insert and relating that back to creamygoodness' explanation and demonstration that used substr to prepend to a string, I thought I'd see if I could use that to simplify things a little. What I came up with is this:

    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 z
+eros
            sv_insert( n, 0, 0, pad, 100 );
            sv_chop( n, SvPVX( n ) + 99 );
            a = SvPVX( n );
            ++l;
        }
    }
    ...
[download]

Whether that could be useful to you I don't know, but if you have the time to cast an eye over it and tell me if you see any obvious fopars... thanks again.