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

##</code><code>##

use strict;
use warnings;

use Benchmark qw( timethese cmpthese );
my $results = timethese(
    5000,
    {   orig => sub {
            delete $P::x{some_key};
            for ( 1 .. 1000 ) {
                $P::x{some_key} ||= ( f(), 1 );
            }
        },
        alt_1 => sub {
            delete $P::x{some_key};
            for ( 1 .. 1000 ) {
                $P::x{some_key} = $P::x{some_key} || ( f(), 1 );
            }
        },
        alt_2 => sub {
            delete $P::x{some_key};
            for ( 1 .. 1000 ) {
                if ( not $P::x{some_key} ) {
                    $P::x{some_key} = 1;
                    f();
                }
            }
        },
    }
);
cmpthese($results);

sub f() {
    #No op.
}

##</code><code>##

Benchmark: timing 5000 iterations of alt_1, alt_2, orig...
     alt_1:  3 wallclock secs ( 3.08 usr +  0.00 sys =  3.08 CPU) @ 1623.38/s (n=5000)
     alt_2:  2 wallclock secs ( 1.29 usr +  0.00 sys =  1.29 CPU) @ 3875.97/s (n=5000)
      orig:  1 wallclock secs ( 1.25 usr +  0.00 sys =  1.25 CPU) @ 4000.00/s (n=5000)
        Rate alt_1 alt_2  orig
alt_1 1623/s    --  -58%  -59%
alt_2 3876/s  139%    --   -3%
orig  4000/s  146%    3%    --