First, I changed your grouping so I could see what was really happening:

% perl -de 0
  DB<1> x "ab" =~ /(a*)((?:ab)*|b*)/
0  'a'
1  ''
[download]

So Perl finds a match quite easily and gives up; fair enough. Regexen are supposed to be greedy but that doesn't include trying a second alternative (something after a veritcal bar) to see if it can match a longer string that way. So lets force Perl to be less lazy by forcing a match to the end of the string:

  DB<2> x "ab" =~ /(a*)((?:ab)*|b*)$/
0  'a'
1  'b'
[download]

Does that answer the question well enough for you?

        - tye (but my friends call me "Tye")