Benchmark: running F_plus, F_sexeger, F_while, P_plus, P_sexeger, P_while, each for at least 5 CPU seconds...
    F_plus:  6 wallclock secs ( 5.38 usr +  0.02 sys =  5.40 CPU) @ 38010.19/s (n=205255)
 F_sexeger:  6 wallclock secs ( 5.19 usr +  0.00 sys =  5.19 CPU) @ 82085.16/s (n=426022)
   F_while:  5 wallclock secs ( 5.23 usr +  0.00 sys =  5.23 CPU) @ 99934.23/s (n=522656)
    P_plus:  6 wallclock secs ( 5.22 usr +  0.00 sys =  5.22 CPU) @ 34659.58/s (n=180923)
 P_sexeger:  7 wallclock secs ( 5.11 usr +  0.00 sys =  5.11 CPU) @ 54039.53/s (n=276142)
   P_while:  6 wallclock secs ( 5.14 usr +  0.00 sys =  5.14 CPU) @ 58260.31/s (n=299458)
              Rate   P_plus   F_plus P_sexeger P_while F_sexeger F_while
P_plus     34660/s       --      -9%      -36%    -41%      -58%    -65%
F_plus     38010/s      10%       --      -30%    -35%      -54%    -62%
P_sexeger  54040/s      56%      42%        --     -7%      -34%    -46%
P_while    58260/s      68%      53%        8%      --      -29%    -42%
F_sexeger  82085/s     137%     116%       52%     41%        --    -18%
F_while    99934/s     188%     163%       85%     72%       22%      --

##</code><code>##

#!/usr/bin/perl

use Benchmark 'cmpthese';

my $X = "a b c d e f g h i j k l ";
my $Y = "a b c d e f g h i j k l";

cmpthese(-5, {
  P_while => sub {
    my $x = $X;
    1 while $x =~ s/\s$//;
  },
  P_plus => sub {
    my $x = $X;
    $x =~ s/\s+$//;
  },
  P_sexeger => sub {
    my $x = reverse $X;
    $x =~ s/^\s+//;
    $x = reverse $x;
  },

  F_while => sub {
    my $x = $Y;
    1 while $x =~ s/\s$//;
  },
  F_plus => sub {
    my $x = $Y;
    $x =~ s/\s+$//;
  },
  F_sexeger => sub {
    my $x = reverse $Y;
    $x =~ s/^\s+//;
    $x = reverse $x;
  },
});