if (length($str) > 1000) {
  substr($str, 1+rindex($str, '.', 1000)) = "";
}

##</code><code>##

#!/usr/bin/perl

use Benchmark 'cmpthese';
use String::Index 'crindex';

my $str = "alphabet. alphabet! alphabet? " x 100;

cmpthese(-5, {
  rcindex => sub {
    my $x = $str;
    substr($x, 1+crindex($str, ".!?", 1000)) = "";
  },
  regex => sub {
    my $x = $str;
    $x =~ s/^(.{1,999}[.!?]).*/$1/;
  },
  rxsubstr => sub {
    my $x = $str;
    $x =~ /^.{1,999}[.!?]/ and substr($x, $+[0]) = "";
  },
});

__END__
             Rate    regex rxsubstr  rcindex
regex     42520/s       --     -43%     -66%
rxsubstr  75202/s      77%       --     -40%
rcindex  125559/s     195%      67%       --