(Side :) note also that the regex expression  (?![b])+ is problematic in the quoted match because it asks the regex engine to match a zero width assertion as many times as possible. Perl would have warned you about this if you had asked it to by enabling warnings:

c:\@Work\Perl\monks>perl -wMstrict -le
"$_ = 'aaab';
 print qq{Matched 1 => '$_'} if /^[a]{2,}(?![b])+/;
 print qq{LastMatch1: '$&'};
"
(?![b])+ matches null string many times in regex; marked by <-- HERE
 in m/^[a]{2,}(?![b])+ <-- HERE / at -e line 1.
Matched 1 => 'aaab'
LastMatch1: 'aa'
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Update: "Possessive" quantifiers were added with Perl version 5.10. Prior to that version and to date, the same functionality can be had with the independent or "atomic" subexpression  (?>pattern)