Of course, another approach would be to common-case all lines before matching...

Adding this other approach to your benchmark test…

C:\>type 634253.pl
#!perl

use strict;
use warnings;

use Benchmark qw( cmpthese );
use English qw( -no_match_vars );

local $OUTPUT_RECORD_SEPARATOR = "\n";

print 'Perl version: ', $PERL_VERSION;

my $s = 'Aid bears out ' x 10_000_000;

print 'Length of string: ', length $s;

cmpthese(-1, {
   'lc'       => sub { lc($s) =~ m{ [aeiou][aeiou]           }xmsg },
   '[Aa][Aa]' => sub {    $s  =~ m{ [AaEeIiOoUu][AaEeIiOoUu] }xmsg },
   '[Aa]{2}'  => sub {    $s  =~ m{ [AaEeIiOoUu]{2}          }xmsg },
   '/i'       => sub {    $s  =~ m{ (?i) [aeiou]{2}          }xmsg },
});

exit 0;

C:\>perl 634253.pl
Perl version: v5.16.2
Length of string: 140000000
              Rate        lc  [Aa][Aa]        /i   [Aa]{2}
lc          5.20/s        --     -100%     -100%     -100%
[Aa][Aa] 2915419/s 56073120%        --      -23%      -25%
/i       3764119/s 72396452%       29%        --       -3%
[Aa]{2}  3877869/s 74584244%       33%        3%        --

C:\>perl 634253.pl
Perl version: v5.16.2
Length of string: 140000000
              Rate        lc   [Aa]{2}        /i  [Aa][Aa]
lc          4.94/s        --     -100%     -100%     -100%
[Aa]{2}  2814401/s 57019656%        --       -8%      -25%
/i       3044909/s 61689750%        8%        --      -19%
[Aa][Aa] 3762832/s 76234868%       34%       24%        --

C:\>
[download]