use strict;
use warnings;
my $text = 'The cat jumped over the dog. Smart cat! He jumped over the dog. The cat jumped over the dog. Smart cat!';
my %seen;
my $longest = '';

while ($text =~ /\s*(.+?[!?.])/g) {
   $longest = $1  if $seen{$1}++ && length $1 > length $longest;
}
print $longest,$/;