The sa-learn man page specifies the file argument for folders to hold one folder
per line with an optional ham: or spam: prefix.

However, Mail::SpamAssassin::ArchiveIterator.pm, on line 413, expects three 
fields: $class, $format, and $rawloc. $format isn't even mentioned in the man
page and it would appear that the ham:/spam: prefixes don't seem all that optional
either. To make this line work, I need to change the lines in the --folders file
to something like "ham:dir:~/.maildir/My dir/cur/".

To fix it, I simply changed line 413 into:
  my ($class, $rawloc) = split(/:/, $target, 3);
  my $format = 'detect';  

I haven't attached a diff, because I modified the original file ...