if ($hash{$x} == 1)
{
     # always slower than if($hash{$x})
     # or if(defined($hash{$x}))
}

if ($hash{$x})
{
     # slower than if(defined($hash{$x}))
     # when $hash{$x} is undefined

     # faster than if(defined($hash{$x}))
     # when $hash{$x} is defined
}

##</code><code>##

#!/usr/bin/perl

use strict;
use Benchmark;

my @list = (10,11,12,13,14,15,17,18,22,23,24,25,26,27,28,29,30,31,32,33,34,35 );
my %hash;
@hash{@list} = (1) x @list;

timethese(1000000, {
        'Hash Defined Miss' => sub {
                if (defined($hash{1})) {
                        #nop
                }
        },
        'Hash Defined Hit' => sub {
                if (defined($hash{10})) {
                        #nop
                }
        },
        'Hash Assigned Miss Truth' => sub {
                if ($hash{1}) {
                        #nop
                }
        },
        'Hash Assigned Miss Equality' => sub {
                if ($hash{1} == 1) {
                        #nop
                }
        },
        'Hash Assigned Hit Truth' => sub {
                if ($hash{10}) {
                        #nop
                }
        },
        'Hash Assigned Hit Equality' => sub {
                if ($hash{10} == 1) {
                        #nop
                }
        }
});


##</code><code>##

Benchmark: timing 1000000 iterations of Hash Assigned Hit Equality, Hash Assigned Hit Truth,
    Hash Assigned Miss Equality, Hash Assigned Miss Truth, Hash Defined Hit, Hash Defined Miss...
Hash Assigned Hit Equality:  5 wallclock secs ( 1.52 usr +  0.00 sys =  1.52 CPU) @ 657894.74/s (n=1000000)
Hash Assigned Hit Truth:  3 wallclock secs ( 1.29 usr + -0.01 sys =  1.28 CPU) @ 781250.00/s (n=1000000)
Hash Assigned Miss Equality:  3 wallclock secs ( 1.30 usr +  0.00 sys =  1.30 CPU) @ 769230.77/s (n=1000000)
Hash Assigned Miss Truth:  1 wallclock secs ( 1.15 usr +  0.01 sys =  1.16 CPU) @ 862068.97/s (n=1000000)
Hash Defined Hit:  1 wallclock secs ( 1.41 usr +  0.00 sys =  1.41 CPU) @ 709219.86/s (n=1000000)
Hash Defined Miss:  2 wallclock secs ( 0.95 usr +  0.00 sys =  0.95 CPU) @ 1052631.58/s (n=1000000)