UPDATE: Read Regexes are slow (or, why I advocate String::Index) for a detailed explanation of the general problem with regexes here.

---

There's no need to use a regex:

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

See the rindex function's docs. It returns the last location of the substring (here, ".") in the string ($str). We're telling it to start looking at the 1000th character (and work backwards).

If you want to allow various punctuation, might I suggest my String::Index module? It's faster than the typical regex solution and a hybrid regex/substr solution.

#!/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%       --
[download]

String::Index gives you four functions that are crosses between Perl's index() function and C's strpbrk() function.

(I need to fix the docs or the module a tad. The function is 'crindex', but I have 'rcindex' somewhere.)