if(not defined($coord_{$key1}{$key2}[2]))
 {
   $coord_{$key1}{$key2}[2] = 0;
 }

##</code><code>##

$coord_{$key1}{$key2}[2] = 0 unless defined $coord_{$key1}{$key2}[2];

##</code><code>##

$coord_{$key1}{$key2}[2] ||= 0;