Re: understanding this syntax

[molecules]

I don't know if the question was edited, but at this moment the question says "Why do we have parentheses around FOO/BAR in the hash?".

Hash assignment takes a list. The following shows that a lack of parentheses makes a difference:

use strict;
use warnings;
use constant FOO => 10; 
use constant BAR => 20; 
use Data::Dump qw/ dump /;

my %BAZ = ( 
    FOO() => 1,
    BAR() => 1
);

my %BOO = ( 
    FOO => 1,
    BAR => 1
);

my %BAH = FOO => 1, BAR => 1;

print 'BAZ: ' . dump(\%BAZ) . "\n";
print 'BOO: ' . dump(\%BOO) . "\n";
print 'BAH: ' . dump(\%BAH) . "\n";

This prints:

Useless use of a constant (BAR) in void context at temp/hash_test.pl line 17.
Odd number of elements in hash assignment at temp/hash_test.pl line 17.
BAZ: { 10 => 1, 20 => 1 }
BOO: { BAR => 1, FOO => 1 }
BAH: { FOO => undef }

Both comma operators (i.e. " =>" and " ,") have lower precedence than the assignment operator " =". Thus the assignment operator takes only the first argument to its right, which for the hash %BAH is the value for the constant FOO. It doesn't even "see" anything to the right of the first fat comma  => or even the comma itself.

Using parentheses around everything to the right of the assignment operator creates a list that the assignment operator in turn uses to create the hash.

Perl is an "Operator-oriented language" as chromatic mentions in his blog post Perl and the Least Surprised.