my @keys = grep /^t/, keys %hash;
my %newhash;
@newhash{@keys} = @hash{@keys};

##</code><code>##

#!perl -w

use strict;

use Benchmark;

Benchmark->import(qw/cmpthese/) if $^V;

my $time = shift || 5;
my $size  = shift || 100;

my %hash;

for (0 .. $size) {
  if ($_ < $size / 4) {
    $hash{"t$_"} = 1;
  } else {
    $hash{"a$_"} = 1;
  }
}

my %bms = (
           grep_map    => sub { my @keys = grep /^\t/, keys %hash;
                                my %newhash = map { ($_, $hash{$_}) } @keys;
                              },
           just_map    => sub { my %newhash = map /^\t/ ?
                                  ( $_, $hash{$_} ) :
                                  (), keys %hash;
                              },
           grep_slice  => sub { my @keys = grep /^\t/, keys %hash;
                                my %newhash;
                                @newhash{@keys} = @hash{@keys};
                              },
          );

if ($^V) {
  cmpthese(-$time, \%bms);
} else {
  timethese(-$time, \%bms);
}

__END__

% perl bm.pl
Benchmark: running grep_map, grep_slice, just_map, each for at least 5 CPU seconds...
  grep_map:  6 wallclock secs ( 5.29 usr +  0.01 sys =  5.30 CPU) @ 4034.15/s (n=21381)
grep_slice:  6 wallclock secs ( 5.28 usr +  0.00 sys =  5.28 CPU) @ 4047.73/s (n=21372)
  just_map:  6 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 3990.06/s (n=20868)
             Rate   just_map   grep_map grep_slice
just_map   3990/s         --        -1%        -1%
grep_map   4034/s         1%         --        -0%

% perl/bin/perl bm.pl 5 1000
Benchmark: running grep_map, grep_slice, just_map, each for at least 5 CPU seconds...
  grep_map:  6 wallclock secs ( 5.26 usr +  0.00 sys =  5.26 CPU) @ 412.17/s (n=2168)
grep_slice:  6 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @ 412.95/s (n=2168)
  just_map:  6 wallclock secs ( 5.29 usr +  0.00 sys =  5.29 CPU) @ 398.87/s (n=2110)
            Rate   just_map   grep_map grep_slice
just_map   399/s         --        -3%        -3%
grep_map   412/s         3%         --        -0%
grep_slice 413/s         4%         0%         --