How to assign variable default values?

The first option is the most used and more understandable to read. I would recommend to stick to it.
Showed colleagues the second option... many did not realize it. So it is better not to use if you work in a team

Personally, I more enjoyable the second option:
\r
\r

argument = argument || 'value';

\r
It is a pity that in JS it is impossible to do so (as in Ruby):
\r
\r

argument ||= 'value';

IMHO the first, because it is more "native"

You gave two equally good options. The first is the most versatile. The second is more ategorization. Choice someone like that.
One of the options (I try to do always) — initialization of variables. To complete the actions describe variables and assign them default values. If the appropriate parameters are set, the variable values are updated.
If you do not assign the argument values to certain variables, and directly appeal to these arguments, the second variant of the record, in my opinion, more beautiful. And the first one is more readable for the person who is poorly familiar with additional language constructs like the 2nd option. plus the second option can generate more compact code, but hardly on the General background of such savings that will give :)
Type options 2 I personally try to only use if the arguments in the expression is rather short. Too bulky design illegible. Forget, then he will understand what is meant by that :)

In Perl 5.10+, by the way, there is a special operator for this: $arg // "value" — the right part is not evaluated when the Boolean falsity, and when undefined (undef) left. Accordingly, there is such construction: $arg //= "value"; — short and correct way to solve the problem.

Yes, do not bother you.
Use the one like in your own scripts and the one that is already used in alien.

in the Treasury
!isset($ar) && $ar = 1;