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")