use Benchmark qw(:all);

cmpthese ( 5000000, {
  'string' => 'string()',
  'hash' => 'hash()',
  'list2string' => 'list2string()',
});

# other subs as before

sub list2string {
  my $str = join( ' ', qw/1689 9679 2792 2514 5472 1520 9342 5544 1268 0165 1979 7314 2101 7221 9539 3882 1812/ );
  my $test = 2101;
  if ($str =~ $test){
    # do stuff
  }
}

__END__

                 Rate                         hash list2string      string
                 hash          62406/s          --        -88%        -95%
                 list2string  530223/s        750%          --        -61%
                 string      1355014/s       2071%        156%          --