my $x = some_sub();
if ( $x > 2 ) {
    # do something here
}

##</code><code>##

if ( defined $x && $x > 2 ) {}

##</code><code>##

#!/usr/bin/perl
use Benchmark;
use strict;

my ( @nums, $x );

for (1..1000) {
    my $r = int(rand(5));
    push @nums, $r > 0 ? $r : undef;
}

timethese(-10, {
    Test_defined => 'for (@nums) { if ( defined $_ and $_ > 2 ) { $x++ }}',
    Do_not_test  => 'for (@nums) { if ( $_ > 2 ) { $x++ }}'
});

##</code><code>##

Benchmark: running Do_not_test, Test_defined, each for at least 10 CPU seconds...
Do_not_test: 11 wallclock secs (10.01 usr +  0.00 sys = 10.01 CPU) @ 365830.27/s (n=3661961)
Test_defined:  8 wallclock secs (10.02 usr +  0.00 sys = 10.02 CPU) @ 363485.63/s (n=3642126)