Dear Monks,

I have a little problem:

use warnings;
use 5.010;

for my $num (1 .. 5) {
    my $a = $num;

    {say "in block: $a"};

    sub test {
        say "in function: $a";
    }

    test()
}

--output:--
in block: 1
in function: 1
in block: 2
in function: 1
in block: 3
in function: 1
in block: 4
in function: 1
in block: 5
in function: 1
[download]

I assume a closure is what's causing the sub to print the same output over and over again. However, the only thing I can find in the docs about closures is in relation to anonymous subs, yet this program is using a named sub.

Is the closure problem an artifact of perl trying to be efficient and not redefining the function every time through the loop? Why don't the docs mention that a named sub can create a closure? Could someone give a blow by blow description of what's happening?

That closure problem sprang up when sorting a hash inside a loop. The following program sorts two hashes by their values, which are integers. The hashes are stored in an array, so a for loop is used to step through the array. Then one hash is sorted each time through the loop, and its results are immediately displayed:

use strict;
use warnings;
use 5.010;

my %h1 = (
    'a' => 5,
    'b' => 8,
    'c' => 1
);

my %h2 = (
    'a' => 200, 
    'b' => 150,
    'c' => 100
);

my @AoH = (\%h1, \%h2);

for my $href (@AoH) {
    my %hash = %$href;  

    my @sorted_keys = sort by_val keys %hash;

    sub by_val {
        $hash{$a} <=> $hash{$b} 
    };

    for my $key (@sorted_keys) {
        say "$key = $hash{$key}";
    }

    say "=" x 20;
}

--output:--
c = 1
a = 5  
b = 8  #ok, the first hash is sorted perfectly.
====================
c = 100
a = 200 
b = 150  #but what happened here?
====================
[download]

The problem can be cured by using a block with sort instead of defining a sub. This example shows that you should not be indifferent to using a block v. defining a sub when using sort().

edit: I also wanted to ask about this variation:

use strict;
use warnings;
use 5.010;

for my $num (1 .. 5) {

    say "start of for loop: $num";

    {say "in block: $num"};

    sub test {
        say "in function: $num";
    }

    test()
}

--output:--
start of for loop: 1
in block: 1
Use of uninitialized value $num in concatenation (.) or string at 3per
+l.pl line 11.
in function: 
start of for loop: 2
in block: 2
Use of uninitialized value $num in concatenation (.) or string at 3per
+l.pl line 11.
in function: 
..
..
[download]

In that variation, the sub can't see the my variable $num. Why?

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