Booking.com Software Developer (Willing to Learn Perl) interview questions

Never had such before. Instead of just giving a task (as an exam) and wait you cracking it they told about a problem and we were working on it together. The whole process included: -coding task on time online. -mobile short talk to HR -online technical interview with 2 team leads -onsite interview. 5 hours, 3 rounds.

After I applied this position, got an Email and need to finish online code test on HackerRank. The test contains 6 coding tests with 3 difficult, 2 medium and 1 easy problem( Just choose the one you can complete in 1 hour) . Then I got call from HR and confirmed the time of technical review. Then, the technical phone review with two developer. The question is there is an endless stream, you can get characters from this stream one by one. How to identify a word from this stream? How to handle the case if the first character of this word appeared several times in this word? For example, grep "interview" from "intervinterview". How to find thousands of word from this string?