It's customary to show what you've tried and to explain where you're stuck. Providing actual sample input and output is also helpful. The specification you have provided is incomplete; for example, what output do you want if the string is "aaa"? Are overlaps included or excluded from your result set?

But most importantly, this site is about helping people to understand and grow as Perl programmers. Questions that don't demonstrate an interest in that process are often off-topic here.