hmm.. yup. after much fiddling with the tests, I couldn't get qr to match /o. My conclusion is:
if you've got a regex containing a variable that will not change, use /o.
if you need to loop through multiple regexes, then you can't use /o, so compile them with qr.
the option to that is to have multiple tests using /o which I think would still be faster.. ok off to test..

again, hmmm..

use Benchmark;

my @words = map { chomp; $_ } (<DATA>);

my $alpha = '[a-zA-Z]';
my $alnum = '[a-zA-Z0-9]';

my @qr = ( qr/^$alpha/, qr/$alnum+$/ );

timethese(500000, {
   'With /o'    => \&testsub,
   'qr'    => \&testsubqr,
});

sub testsub{
  my $count = 0;
  foreach(@words){
    $count += testsubb($_);
  }
}

sub testsubb {
  my $word = $_[0];
  return unless $word =~ /^$alpha/o;
  return unless $word =~ /$alnum+$/o;
  return 1;
}

sub testsubqr{
  my $count = 0;
  foreach(@words){
    $count += testsubbqr($_);
  }
}

sub testsubbqr {
  my $word = $_[0];
  foreach(@qr){
    return unless $word =~ $_;
  }
  return 1;
}

Benchmark: timing 500000 iterations of With /o, qr...
   With /o: 12 wallclock secs (13.13 usr +  0.01 sys = 13.14 CPU) @ 38
+051.75/s (n=500000)
        qr: 18 wallclock secs (18.90 usr +  0.01 sys = 18.91 CPU) @ 26
+441.04/s (n=500000)

Benchmark: timing 100000 iterations of With /o, qr...
   With /o:  3 wallclock secs ( 2.64 usr +  0.00 sys =  2.64 CPU) @ 37
+878.79/s (n=100000)
        qr:  4 wallclock secs ( 3.78 usr +  0.00 sys =  3.78 CPU) @ 26
+455.03/s (n=100000)
[download]

I think I'm done defending qr..

ok, back to work.. nothing to see here...

update: adding these tests shows that the loop is adding more time than qr saves, but /o is still quicker.. so where does that leave us?

sub testsubqr2{
  my $count = 0;
  foreach(@words){
    $count += testsubbqr2($_);
  }
}

sub testsubbqr2 {
  my $word = $_[0];
  return unless $word =~ $qr[0];
  return unless $word =~ $qr[1];
  return 1;
}

Benchmark: timing 100000 iterations of With /o, qr, qr2...
   With /o:  3 wallclock secs ( 2.63 usr +  0.00 sys =  2.63 CPU) @ 38
+022.81/s (n=100000)
        qr:  4 wallclock secs ( 3.76 usr +  0.00 sys =  3.76 CPU) @ 26
+595.74/s (n=100000)
       qr2:  3 wallclock secs ( 2.98 usr +  0.00 sys =  2.98 CPU) @ 33
+557.05/s (n=100000)
[download]

cheers,

J