This loses on your golf requirement, but its better on being more correct in locale-specific environments:

my $str = "  abc def123abc def";
print "[$_]\n" for split /
  \s+|
  (?<=[[:alpha:]])(?=[[:digit:]])|
  (?<=[[:digit:]])(?=[[:alpha:]])
/ix, $str;
[download]

I think doing what you ask is somewhat doable, but it would just be more of a mess. And turning the problem inside-out as merlyn suggests is a much better answer anyway :-)