You're looking for defined. Examples: 

defined $num or $num = 1;
$num = 1 if ! defined $num;

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In Perl 5.10, there's a // operator for testing if something is defined. 

$num // $num = 1;
$num //= 1;

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In reply to Re: Checking the value of an undefined value by kyle
in thread Checking the value of an undefined value by Anonymous Monk

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