There's no difference. Both declare a hash with zero key/value pairs: 

$ perl -Mstrict -Mwarnings -E '
    my %x;      say scalar keys %x; say scalar values %x;
    my %y = (); say scalar keys %y; say scalar values %y;
'
0
0
0
0

[download]

Including an assignment has some overhead: typically negligible but may be significant in looping code. 

#!/usr/bin/env perl

use strict;
use warnings;

use Benchmark qw{cmpthese};

cmpthese -1 => {
    no_assignment => sub { my %hash },
    assignment    => sub { my %hash = () },
};

[download]

Output:

                    Rate    assignment no_assignment
assignment     6672755/s            --          -60%
no_assignment 16770827/s          151%            --

[download]

I wouldn't necessarily consider one form to be "more correct" than the other.

I generally use the "my %hash_data;" form.

[Minor Update: I removed "use autodie;" from the benchmark code as it wasn't necessary (it was an artefact from the last use of this script which I often rework for example code); retested; much the same results.]

-- Ken

In reply to Re: Is there a difference in this declaration? by kcott
in thread Is there a difference in this declaration? by Anonymous Monk

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