Post your code for this Benchmark and I'll show you where it went wrong. I'd bet on the variable you tested against not being in scope inside the benchmark sub/evals.

My results, Linux on an IBM Netfinity (Intel)

Benchmark: running regexp, transl, each for at least 10 CPU seconds...
    regexp: 10 wallclock secs (10.59 usr +  0.00 sys = 10.59 CPU) @ 37
+274.69/s (n=394739)
    transl: 13 wallclock secs (10.46 usr +  0.00 sys = 10.46 CPU) @ 31
+3981.07/s (n=3284242)
           Rate regexp transl
regexp  37275/s     --   -88%
transl 313981/s   742%     --
[download]

That is a significant differential there for this simple task. A full regexp engine is a big thing to throw at a lightweight string scan. My benchmark code follows:

use strict;
use Benchmark qw(cmpthese);
use vars qw( $x );

$x = 'This-is-a-test-string-I-just-typed-in-for-fun';

cmpthese (-10, {
    'transl' => '$x =~ tr/-/_/; $x =~ tr/_/-/;',
    'regexp' => '$x =~ s/-/_/g; $x =~ s/_/-/g;',
} );
[download]

Oh yeah, I sure am happy Benchmark exists too. =)

Doh! Update: that assignment was:
$x = 'This_is_a_test_string_I_just_typed_in_for_fun';
It wasn't result impacting, just stupid since it no-ops half my test. Interestingly, if I change the string to one with spaces rather than the '-' or '_' I wind up with regexp being 50-60% faster at doing nothing but scanning with no changes...

--
$you = new YOU;
honk() if $you->love(perl)

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