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

In reply to Re: use of uninitialized values by nothingmuch
in thread use of uninitialized values by Baz

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