$s = "Claims revolve around the longest printed sentence, as there is no absolute limit on the length of a written English sentence; a sentence describing successive reading, for example, could be infinitely long, and one concatenating clauses with grammatical conjunctions such as and could go on as long as material may be supplied; which makes any claim made for the longest sentence trivial to overturn. then some other stuff here.";;

$s =~ s[^.{1,400}\.\K.*?$][];;

print length $s;;
401

print $s;;
Claims revolve around the longest printed sentence, as there is no absolute limit on the length of a written English sentence; a sentence describing successive reading, for example, could be infinitely long, and one concatenating clauses with grammatical conjunctions such as and could go on as long as material may be supplied; which makes any claim made for the longest sentence trivial to overturn.