Perl defined-or in a list context, why a scalar?

The behavior you are observing is the intended one. This is documented in perlop, in the section Logical Defined-Or:

EXPR1 // EXPR2 returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself).

And, perldoc later provides the following example:

In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:

@a = @b || @c;            # This doesn't do the right thing
@a = scalar(@b) || @c;    # because it really means this.
@a = @b ? @b : @c;        # This works fine, though.

// must evaluate its left-hand side operator in scalar context since it requires a scalar to evaluate for defined-ness.

@a returns the number of elements the array contains in scalar context.

So @a // ... always returns the number of elements in @a (since all numbers are defined values).

The concepts of defined-ness or truth-ness aren't defined for a sequence of scalars ("list") as a whole. They only apply to individual scalars. As such, //, &&, and, ||, or, !, not and ?: require a scalar from their left-most operand, so they evaluate it in scalar context. xor needs to test the truth of both of its operands, and thus evaluates both in scalar context.

$ perl -M5.010 -e'
   sub cx { print wantarray ? " list  " : " scalar"; $_[0] }

   print "// ";  @a =   cx($u) //  cx();     say "";
   print "&& ";  @a =   cx(0)  &&  cx();     say "";
   print "and";  @a = ( cx(1)  and cx() );   say "";
   print "|| ";  @a =   cx(0)  ||  cx();     say "";
   print "or ";  @a = ( cx(0)  or  cx() );   say "";
   print "xor";  @a = ( cx(0)  xor cx(0) );  say "";
   print "!  ";  @a =   !   cx();            say "";
   print "not";  @a =   not cx();            say "";
   print "?: ";  @a =  cx() ? 0    : 0;
                 @a =  1    ? cx() : 0;
                 @a =  0    ? 0    : cx();   say "";
'
//  scalar list
&&  scalar
and scalar list
||  scalar list
or  scalar list
xor scalar scalar
!   scalar
not scalar
?:  scalar list   list

You might be under the incorrect impression that @a always returns a list of its elements, and that this gets coerced into a count when evaluated in scalar context. But that's not the case.

There's simply no such thing as a list. When we say "evaluates to a list" or "returns a list", we simply mean "adds zero or more scalar to the stack". There's absolutely no difference between return a scalar and returning "a list of one scalar". Both refer to adding a scalar to the stack.

Since there's no list data structure being returned, there's nothing that can be magically coerced to a scalar in scalar context; its up to every operator to return a single scalar when evaluated in scalar context. This permits each operator to choose what they want to return in scalar context. It varies greatly.

In short, @a returns the number of elements in scalar context, not the elements it contains like it does in list context.

         +-------------------------- (Scalar context) Returns 3
         |            +------------- (List context) Returns three scalars
         |            |
      vvvv     vvvvvvvv
() =  @row // (10,20,30);

         +-------------------------- (Scalar context) Returns 3
         |      +------------------- (List context) Returns three scalars
         |      |           +------- (List context) Returns three scalars
         |      |           |
      vvvv   vvvv    vvvvvvvv
() =  @row ? @row : (10,20,30);

Finally, let's analyze @row // ....

We've already established that @row is evaluated in scalar context in the above, and that it returns the number of elements in the array in array when it does.

Well, numerical values are necessarily not undef, so they are all defined. So that means that the right-hand size of @row // ... will never get evaluated. You might as well have written scalar(@row).

defined(@array) is a bit of a weird construct and Perl is trying to get rid of it. From perldoc -f defined:

Use of "defined" on aggregates (hashes and arrays) is no longer supported. It used to report whether memory for that aggregate had ever been allocated.

That's not testing if it currently has any elements! To check for size, simply use the array variable in scalar context (such as a conditional):

if( @array ) { ... }

And, Dada has already explained what the defined-or operator does, which adds its own twist to it.

// and || first impose scalar context on their left side argument. // checks whether that argumend is defined. || checks whether that argument is "not false". false in Perl is any of the values undef, 0, "0", or "".

The scalar value of an array is always defined so you can't use that in a check the way you did. @row // (10,20,30) is the same as defined(scalar(@row)) or (10,20,30).

For a longer discussion of context, see the documentation for the Want module.