return [ map { $uid2msg{$_} } @$uids ];

##</code><code>##

return [ @uid2msg{ @$uids } ];

##</code><code>##

Benchmark: running hashslice, hashslice_pre, original, original_pre for at least 5 CPU seconds...
 hashslice:  5 wallclock secs ( 5.29 usr +  0.00 sys =  5.29 CPU) @ 97.35/s (n=515)
hashslice_pre:  5 wallclock secs ( 5.30 usr +  0.01 sys =  5.31 CPU) @ 1757.63/s (n=9333)
  original:  5 wallclock secs ( 5.30 usr +  0.00 sys =  5.30 CPU) @ 91.51/s (n=485)
original_pre:  5 wallclock secs ( 5.33 usr +  0.00 sys =  5.33 CPU) @ 817.07/s (n=4355)

##</code><code>##

Benchmark: running hashslice, hashslice_pre, original, original_pre for at least 5 CPU seconds...
 hashslice:  5 wallclock secs ( 5.04 usr +  0.04 sys =  5.08 CPU) @  7.28/s (n=37)
hashslice_pre:  5 wallclock secs ( 5.27 usr +  0.01 sys =  5.28 CPU) @ 93.56/s (n=494)
  original:  5 wallclock secs ( 5.08 usr +  0.01 sys =  5.09 CPU) @  6.68/s (n=34)
original_pre:  6 wallclock secs ( 5.37 usr +  0.00 sys =  5.37 CPU) @ 46.93/s (n=252)

##</code><code>##

use Benchmark;

my $UID = 0;

my $uids = [];
my $msgs = [];

for (1..10000)
{
  UO->new;
}

my %pre_uid2msg = map { $_->uid => $_ } @$msgs;
timethese(-5,
  {
    original => sub {
      my %uid2msg     = map { $_->uid => $_ } @$msgs;
      return [ map { $uid2msg{$_} } @$uids ];
    },
    hashslice => sub {
      my %uid2msg     = map { $_->uid => $_ } @$msgs;
      return [ @uid2msg{ @$uids }];
    },
    original_pre => sub {
      return [ map { $pre_uid2msg{$_} } @$uids ];
    },
    hashslice_pre => sub {
      return [ @pre_uid2msg{ @$uids }];
    } 
  }
);

package UO;

sub new
{
  $UID += rand(1000);
  my $self = bless {uid => $UID}, shift();
  push(@$uids, $UID);
  push(@$msgs, $self);

  return $self;
}
 
sub uid
{
  my $self = shift;
  return $self->{uid};
}