Let's find out... (I like your idea of using modulus on the rotate parameter first, but I left that out of the benchmark so that the comparison would be fair.)

#!/usr/local/bin/perl -w

use Benchmark;

@long = ('a' x 100, 45);
@short = ('a' x 10, 7);

timethese( -10, {
                 L_ariels => sub { ariels(@long) },
                 L_merlyn => sub { merlyn(@long) },
                 L_nashdj => sub { nashdj(@long) },
                 S_ariels => sub { ariels(@short) },
                 S_merlyn => sub { merlyn(@short) },
                 S_nashdj => sub { nashdj(@short) },
                }
          );

sub ariels {
  my ($str, $rot) = @_;
  scalar reverse ((reverse substr($str,0,$rot)) .
                  (reverse substr($str,$rot))
                 );
}
 
sub merlyn {
  my ($str, $rot) = @_;
  substr($str,$rot) . substr($str, 0, $rot);
}
 
sub nashdj {
  my ($str, $rot) = @_;
  $str =~ s/(.{$rot})(.*)/$2$1/s;
  $str;
}
[download]

And the results:

Benchmark: running L_ariels, L_merlyn, L_nashdj, S_ariels, S_merlyn, S_nashdj,
each for at least 10 CPU seconds...
  L_ariels: 13 wallclock secs ( 9.97 usr +  0.04 sys = 10.01 CPU) @ 71186.51/s (n=712577)
  L_merlyn: 10 wallclock secs ( 9.95 usr +  0.06 sys = 10.01 CPU) @ 92000.70/s (n=920927)
  L_nashdj: 12 wallclock secs ( 9.95 usr +  0.06 sys = 10.01 CPU) @ 25479.52/s (n=255050)
  S_ariels: 11 wallclock secs ( 9.99 usr +  0.07 sys = 10.06 CPU) @ 89239.76/s (n=897752)
  S_merlyn: 13 wallclock secs ( 9.95 usr +  0.05 sys = 10.00 CPU) @ 100328.30/s (n=1003283)
  S_nashdj: 10 wallclock secs (10.27 usr +  0.10 sys = 10.37 CPU) @ 28213.11/s (n=292570)

So, using substr is in fact quite a bit faster than using a regex. I think this is not surprising: the substitution being done is too simple to make the overhead of the regex engine worthwhile.