Performance regression in perl with precompiled regexps

Some time ago I wrote about preparing regexpes earlier for cases where you can not use /o modifier. For example you have:

    my $re = qr{\d+};

And often you need to match:

    $str =~ /^$re$/;

You can not use /o modifier if $re may change. In the blog post I've described precompiling and showed that it works as fast as regexp with /o modifier:

    my $re = qr{...};
    my $re_prepared = /^$re$/;
    $str =~ $re_prepared;

Guess my surprize when I executed the same script with perl 5.14.1. Any method is at least 2 times faster than prepared, even ... =~ /^$re$/!

Results for 5.8.9:

    $ perl5.8.9 dynamic_regexp_with_o_effect.pl
                         Rate      no_o         o no_o_pre no_o_pre_braces    static
    no_o            1581903/s        --      -10%     -25%            -32%      -50%
    o               1764430/s       12%        --     -16%            -24%      -44%
    no_o_pre        2104366/s       33%       19%       --             -9%      -34%
    no_o_pre_braces 2323549/s       47%       32%      10%              --      -27%
    static          3174754/s      101%       80%      51%             37%        --

Results for 5.10.0 and newer:

    $ perl5.10.0 dynamic_regexp_with_o_effect.pl
                         Rate no_o_pre no_o_pre_braces      no_o         o    static
    no_o_pre         873813/s       --             -0%      -52%      -68%      -75%
    no_o_pre_braces  877714/s       0%              --      -52%      -68%      -75%
    no_o            1816840/s     108%            107%        --      -34%      -49%
    o               2759410/s     216%            214%       52%        --      -22%
    static          3542486/s     305%            304%       95%       28%        --