See the below examples and find the differences

Case 1:

perl -e 'print map {++$count," : ",$_,"\n"}split /(a|b)+/,"a12cdabab"'
1 : 
2 : a
3 : 12cd
4 : b
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Case 2:

perl -e 'print map {++$count," : ",$_,"\n"}split /((?:a|b)+)/,"a12cdab
+ab"'
1 : 
2 : a
3 : 12cd
4 : abab
[download]

Case 3:

perl -e 'print map {++$count," : ",$_,"\n"}split /(?:a|b)+/,"a12cdabab
+"'
1 : 
2 : 12cd
[download]