Checking if a scalar has a defined value (and assigning a default value if it hasn't) is quite common. Unfortunately, the constructs

$a = $a ? $a : "default_value";
[download]

or

$a ||= "default_value";
[download]

don't work if the value of $a is something that returns false in a boolean context, like 0 or "". For now,

$a = defined $a ? $a : "default_value";
[download]

is the best way to write this, and since it's such a common idiom, Perl 6 will allow you to say

$a //= "default_value";
[download]

whenever Perl6 will come out.