P_rindex => sub {
    my $x = $X;
    while (1) { my $i = length($x); (rindex($x," ") == $i ? chop : last) };
  },
  F_rindex => sub {
    my $x = $Y;
    while (1) { my $i = length($x); (rindex($x," ") == $i ? chop : last) };
  },

##</code><code>##

Benchmark: running F_plus, F_rindex, F_sexeger, F_while, P_plus, P_rindex,P_sexeger, P_while, each for at least 5 CPU seconds...
    F_plus:  6 wallclock secs ( 5.15 usr +  0.00 sys =  5.15 CPU) @ 37600.97/s(n=193645)
  F_rindex:  6 wallclock secs ( 5.45 usr +  0.00 sys =  5.45 CPU) @ 59183.30/s(n=322549)
 F_sexeger:  7 wallclock secs ( 5.24 usr +  0.00 sys =  5.24 CPU) @ 59187.60/s(n=310143)
   F_while:  7 wallclock secs ( 5.29 usr +  0.00 sys =  5.29 CPU) @ 69741.02/s(n=368930)
    P_plus:  5 wallclock secs ( 5.46 usr +  0.00 sys =  5.46 CPU) @ 33136.08/s(n=180923)
  P_rindex:  6 wallclock secs ( 5.31 usr +  0.00 sys =  5.31 CPU) @ 60743.69/s(n=322549)
 P_sexeger:  5 wallclock secs ( 5.36 usr +  0.00 sys =  5.36 CPU) @ 49557.65/s(n=265629)
   P_while:  6 wallclock secs ( 5.09 usr +  0.00 sys =  5.09 CPU) @ 50179.17/s(n=255412)
            Rate P_plus F_plus P_sexeger P_while F_rindex F_sexeger P_rindex F_while
P_plus    33136/s     --   -12%      -33%    -34%     -44%      -44%     -45%-52%
F_plus    37601/s    13%     --      -24%    -25%     -36%      -36%     -38%-46%
P_sexeger 49558/s    50%    32%        --     -1%     -16%      -16%     -18%-29%
P_while   50179/s    51%    33%        1%      --     -15%      -15%     -17%-28%
F_rindex  59183/s    79%    57%       19%     18%       --       -0%      -3%-15%
F_sexeger 59188/s    79%    57%       19%     18%       0%        --      -3%-15%
P_rindex  60744/s    83%    62%       23%     21%       3%        3%       ---13%
F_while   69741/s   110%    85%       41%     39%      18%       18%      15%--