Because your regex contains capturing parens, split will return both the separated bits and the captured separators:

print split ',', 'a,b,c';;
a b c

print split '(,)', 'a,b,c';;
a , b , c
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However, as you have a quantifier applied to the capturing parens, the regex will match multiple consecutive characters matching (a|b)+ but will only retain the last one captured. Hence although the regex matches & captures each of the four characters in 'abab', only the final 'b' will be retained and returned.

print 'abcd' =~ m[(.)+];;
d
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Putting that all together, and the delimiters used to split the string are as indicated by '[]': "[a]12cd[abab]"; but only the last single character of each delimiter is retained, hence:'(a)', '12cd', '(b)'