Perl has a function named undef, which either undefines a value (as in undef @bar), or returns an undefined value ($foo = undef). To check for definedness, you use the function defined:

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

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

$coord_{$key1}{$key2}[2] ||= 0;
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Update:I forgot to say that undef is the 'uninitialized value' - an undefined (not only false but with no value).

-nuffin
zz zZ Z Z #!perl