$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.

##</code><code>##

$s = "The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. A sentence that exactly fills to the 400.  The quick brown fox jumps over the lazy dog.  The quick brown fox jumps over the lazy dog.  The quick brown fox jumps over the lazy dog.";;

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

print length $s;;
401

print $s;;
The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. The quick brown fox jumps over the lazy dog. A sentence that exactly fills to the 400.