It's because of the greedy .+ you have after [A-Z] ... just tweaking that to be [^A-Z] instead of . will work, and gives what you want with that test data. An alternative is to do a substitution, then split on whitespace.

use strict;
use warnings;

while(<DATA>){
  # split regex
  print join ":", grep length, split /(?:(.+?[a-z])([A-Z][^A-Z]+))/g, 
+$_;

  # substition then split method
  s/(?<=[a-z])(?=[A-Z])/ /g;
  print join(":", split ' ', $_), "\n";
}

__DATA__
Genetics
Genomics
phylogeny
allele
ChromosomeLocusLink
geneExpression
RasSignalTransductionPathway
[download]