I can't really answer "why" Larry and crew chose the things they did. But, I might be able to shed a little light.

Let's look at your examples

scalar((1,3,5))     #evaluates to the last element of the list --- 5
if( (5,3,0) )       #evaluates to the last element of the list --- 0, 
+thus evaluates to false
$val=(5,6,7);       #evaluates to the last element of the list --- 7
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These use the comma operator ',' in scalar context, returning whatever is on the right of the comma. In the case of multiple commas, you get the value of the rightmost expression.

There is no list here at all. You just think there is because you have been trained to think of the comma as meaning 'list'.

$ret = (($i, $j, $k)=(5,6,8,9));        #evaluates to the number of el
+ements on the right side list --- 4
if( ($k, $v) = ( 1, 0 ) )               #evaluates to the number of el
+ements on the right side list --- 2, thus evaluates to TRUE
while ( ($key, $value) = each %map )    #still evaluates to the number
+ of elements on the right side hash
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Here we actually have some lists, and thus actual list assignment.

List assignment is scalar context gives us the size of the list that was passed into the assignment. So here $foo = ($bar, $baz) = (1..50); $foo is equal to 50.

In list context you get the contents of the resulting list. So  @foo = () = (1..50) leaves @foo empty.

Why is it this way? I don't know. But I can tell you that getting the size of the assigned from list can be useful.

my $count_foo = () = /foo/g;
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Written a bit more compactly, that construct gets called "the goatse operator": $foo =()= /foo/g; If you don't get the joke, don't look it up.

I hope this helps a bit.

List assignment in a scalar context returns the number of items in the right-hand side because that is extremely useful as a Boolean test, as shown in your own example:

    while( my( $k, $v ) = each %h ) {
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You wouldn't want that loop to terminate early just because one of the values in %h was false.

scalar((1,2,3))

has no list in it. The confusion is that we (English speakers) "list" things by separating them using commas. From Perl's perspective, there is nothing in that statement to provide a list context. "scalar" can take any expression, and just sees comma separators, no matter how many sets of parentheses you surround it with.

on the other hand, has an assignment that forces a list context, so the answer becomes "3".

works the same way. The left side of the assignment is now a list, because of the equals sign. The overall context of any boolean test is scalar, so that an "if list" always provides the element count.

@TGI: You have said that there is no list at all in:

if( (5,3,0) ) 
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I tested below. You are right.

if( (5,3,0) ) is equal to if( 5,3,0 )
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both evaluate to FALSE. I need to understand the definition of list furthur.

@tye and Anonymous Monk : Your comments make sense.

From your patient answers, I think I can draw the conclusion: The reason that list assignment in scalar context will return the number of elements on right side list is for convenience of usage.