First off, sorry, I have no idea what you're trying to do, so I can offer no concrete help.

However, as nobody has answered this question in the more than six hours that have passed since it was posted, and as you have "run out of ideas", why not try YAPE::Regex::Explain?

Something like this:

use YAPE::Regex::Explain;
# Your code as posted
my $regex = YAPE::Regex::Explain->new ( qr/$split_re/ );
print $regex->explain;
[download]

dave