You could try something along these lines (untested): 
m/\A \s* (\w+\s+\w+?.*?) (??{ length($1) >= 50 }) \s* \z/xs;

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(Update: this regex might be very inefficient because both \w+? and .*? might match the same number characters. Use (\w+\s+\w.*?) instead.)

In general that sounds like a problem for which regexes aren't the best solution (split and friends might be better).

In reply to Re: RE question: Sentence with a minimum length by moritz
in thread RE question: Sentence with a minimum length by lima1

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