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
}