For what it's worth, some of this applies equally in many languages such as C and C++. There's not really a concept of definedness in C/C++, but the concept of short circuiting in expressions using ||, and of ternary operators is nearly identical.

But please be warned that the semantics of the || and && operators in C/C++ is significantly different to Perl. For example, this Perl code:

my $var;
$var = 0;
$var = $var || 30;
print "1. var=$var\n";
$var = 0;
$var ||= 30;
print "2. var=$var\n";
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prints:

1. var=30
2. var=30
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While this C++ code:

int var = 0;
var = var || 30;
printf("1. var=%d\n", var);
// var ||= 30;     // oops, syntax error in C++
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prints:

1. var=1
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Why? Because in C/C++, the (short-circuiting) || and && operators always return 0 or 1. Unlike Perl, they do NOT return the last value evaluated.

IMHO, this deliberate Perl change in semantics of these two venerable short-circuiting operators was a clever innovation from Larry, making them more convenient to use, notably when setting default values.