This is closely related to the concept of the default input operator

What are you talking about???

• $_ is not an operator, at least no more so than $x.
• Some functions default to using $_ as an argument, but that is unrelated to this thread.
• $_ has nothing to with input.
• How can an input operator have anything to do with this?

And the rest of the post is no better. So many errors!

sub a{ return 1, 2, 3 } a();

This evaluates what a() returns, in scalar context. $_ gets the last value returned by a(). In this case, $_ is 3, or True if evaluated in Boolean context. If you tested $x = a(), you'd get the same True result, with $x equal to 3.

• That code doesn't provide enough information to determine in which context a() is evaluated. If anything, void context is implied.
• Nothing is assigned to $_.
• If a() is evaluated in scalar context, then it makes no sense to talk about the last value it returns as it only returns one.

sub a{ return 1, 2, 0 }  a();

$_ gets the last value returned by a(), as before. In this case, $_ is 0 in scalar context, or False in Boolean context!

• Nothing is assigned to $_.
• The same value is returned by the comma operator in both "boolean context" and scalar context.

@a = (undef, undef); @a;

This array is evaluated in scalar context because we haven't done anything to force list context. An array, evaluated in scalar context, is always the number of elements it contains.

• That code doesn't provide enough information to determine in which context a() is evaluated. If anything, void context is implied.
• There is no default context.

sub a{ return 1, 2, 0 } ($x,$y) = a();

We're still evaluating what a() returns, but here we've forced list context. That means that $_ is assigned the number of elements in the list returned, not the elements. It doesn't matter what a() returns -- if it returns one or more elements, $_ will be set to non-zero, or True in Boolean context.

• List context isn't being forced, if there is such a thing.
• Nothing is assigned to $_, so most of that is just nonsense.
• The same value is returned by the list assignment operator in both "boolean context" and scalar context.

sub a{ return 1, 2, 0 }  @arr = a();

Exactly the same as above.

• List context isn't being forced, if there is such a thing.
• Nothing is assigned to $_, so most of that is just nonsense.
• The same value is returned by the list assignment operator in both "boolean context" and scalar context.

(undef, undef);

An array is a container for a list. This is NOT an array, and we haven't done anything to force list context, so we have a list of scalars evaluated in scalar context. $_ is set to the last element, which is undef -- or False in Boolean context.

• There is no default context.
• That code doesn't provide enough information to determine in which context a() is evaluated. If anything, void context is implied.
• Nothing is assigned to $_.
• The same value is returned by the comma operator in both "boolean context" and scalar context.

sub a { return undef, undef } ($x, $y) = a(); if(not $x && not $y) { print "False\n" }

I think this is closer to what you're looking for. Whenever you don't specifically lay out the condition you are testing, you are probably testing $_.

No, it's not what he wants.

• It doesn't loop.
• The condition of the "if" statement relies on incorrect assumptions about precedence.
• A truth test is two levels removed from optiomal. (A defined test would be better, but testing whether the iterator returned anything at all would be best.)

I hope this helps.

Pure harm. Good thing the formatting makes it unreadable.