What is a difference while assigning hash in list context?

Question

I have to expressions:

%MON =    months => 1, end_of_month => 'limit';      # months => undef
%MON =  ( months => 1, end_of_month => 'limit' );

Why first expression results only one key months with undef value? What is the difference between them?

Answer 1

See perlop. = has higher precedence than =>

%MON =    months => 1, end_of_month => 'limit'; 

Is equivalent to:

(%MON = "months"), 1, "end_of_month", "limit"

While:

%MON =  ( months => 1, end_of_month => 'limit' );

is:

%MON = ("months", 1, "end_of_month", "limit")

Answer 2

Here's Perl's table of operator precedence (from perlop):

left        terms and list operators (leftward)
left        ->
nonassoc    ++ --
right       **
right       ! ~ \ and unary + and -
left        =~ !~
left        * / % x
left        + - .
left        << >>
nonassoc    named unary operators
nonassoc    < > <= >= lt gt le ge
nonassoc    == != <=> eq ne cmp ~~
left        &
left        | ^
left        &&
left        || //
nonassoc    ..  ...
right       ?:
right       = += -= *= etc.
left        , =>
nonassoc    list operators (rightward)
right       not
left        and
left        or xor

Notice that = has higher precedence than either , or =>. Therefore, you need the parentheses to override the precedence.