Basically I'm looking for a better method than creating a smaller test string with substr(0, 1000).

That looks like micro-optimization. Taking jwkrahn's and GrandFather's propositions:

#!/usr/bin/perl

use Benchmark qw(cmpthese);

$substr = join('',a..j);
$str = $substr x 90 . 'hTmL'. $substr x 2000;
print "length of search string: ",length $str, "\n";
cmpthese(-3, {
    # \A anchores at the beginning of a string, so no ^. See below (de
+merphq)
    #' regex ' => sub { $str =~ /^\A.{0,996}?html/si; },
    ' regex ' => sub { $str =~ /\A.{0,996}?html/si; },
    ' substr' => sub { (substr $str, 0, 1000) =~ /html/i; },
    #'!regex ' => sub { $str =~ /^\A.{0,996}?sgml/si; },
    '!regex ' => sub { $str =~ /\A.{0,996}?sgml/si; },
    '!substr' => sub { (substr $str, 0, 1000) =~ /sgml/i; },
});
__END__
length of search string: 20904
            Rate  regex   substr !substr !regex 
 regex   30996/s      --    -65%    -72%    -79%
 substr  88073/s    184%      --    -21%    -42%
!substr 111975/s    261%     27%      --    -26%
!regex  150866/s    387%     71%     35%      --
[download]

What can we deduce from that? Not much. The efficiency of either method seems to depend on whether the searched pattern is contained in the string. The results may vary with the position the pattern in the string.

More important, even the "slowest" of these searches performs at a rate of ~31000/second. How many searches do you have to do in what time? In what context? How does the rest of your code perform?

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}