I generally use it in-line in cases like this:

printf("The answer is %s\n", defined($ans) ? $ans : "unknown");'

or

$ans = defined($ans) ? $ans : "unknown";

This is left over from C programming days. I stopped at the first known construct. It seems efficient versus this:

printf("The answer is ");
if (defined($ans))
{
    printf("%s\n", $ans);
}
else
{
    printf("unknown\n");
}

I would not have a tendency to use it on multi-line constructs as shown in other examples in this thread because the ? and : tend to get lost.