Select Code to Download

if (not $P::x) {
    $P::x = 1;
    f();
}

use strict;
use warnings;
...
sub f() {
    #No op.
}

Benchmark: timing 5000 iterations of alt_1, alt_2, orig...
     alt_1:  3 wallclock secs ( 3.08 usr +  0.00 sys =  3.08 CPU) @ 16
+23.38/s (n=5000)
...
alt_1 1623/s    --  -58%  -59%
alt_2 3876/s  139%    --   -3%
orig  4000/s  146%    3%    --