This is a case where adding "use warnings" to the code will explain what is going on.

Useless use of hash elem in void context at ./hash line 9.
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Line 9 is:

$line{$a}{$b}{$c};
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and that code isn't doing anything (Note: well, it isn't doing what you think it's doing - see reasonablekeith's reply below for details). What do you think it should do?

You could also try printing the hash contents with Data::Dumper. You'll see that it's still empty and therefore your various calls to keys will all return empty lists.

If you change the line to:

$line{$a}{$b}{$c} = 1;
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then the hash has been given some data to hold and all of your keys spring into existance.

   and that code isn't doing anything. What do you think it should do?

That's not quite true. Perl will autovivify the first two keys. Dumper reveals %line to be...

print Dumper(\%line);

__OUTPUT__

$VAR1 = {
          'A' => {
                   'B' => {}
                 }
        };
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---
my name's not Keith, and I'm not reasonable.

secondly, also always

use strict;
use warnings;
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my $string = "why doesn't this print?";
print $strnig;
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my $string = "A";

# rather than

my $string = A;
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sub A { return 'B' };

# ... 1000 lines of complicated code ...

my $string = A;
print "$string\n"; # this prints "B"
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using warnings will give you handy hints, like when you use a variable but haven't defined a value for it yet. A lot of it's messages can be ignored, but they'll often give you hints to potential problems with your code.

Applying all this to your script gives you...

#/perl/usr/bin
use warnings;
use strict;

my %line;

my $why = "";
my $key_a = 'A';
my $key_b = 'B';
my $key_c = 'C';

$line{$key_a}{$key_b}{$key_c}; # this is line 12

my $i = 1;

for my $loop_key_a (keys %line) {
    for my $loop_key_b (keys %{$line{$loop_key_a}}) {
        for my $loop_key_c (keys %{$line{$loop_key_a}{$loop_key_b}}) {
            print "$loop_key_a $loop_key_b $loop_key_c\n";
        }
    }
}
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Happy Coding!

A lot of [warnings] can be ignored

In general, warnings should NOT be ignored. There are certain exceptions (e.g., in a CGI script it is common to interpolate an input into link URLs without checking whether it is defined, so that if the user once specifies it it is subsequently preserved), but even there you should only ignore these warnings if you are certain you understand fully what is causing them, and even then it's good practice to clean them up, to make life easier when maintaining your code later (since noisy output can obscure real issues).

If you get into the habbit of ignoring warnings, you're going to end up with a big mess down the road.

1. Avoid using the scalar variables $a and $b. These are special global variables that are used by sort
2. If you are new to Perl, you should always, always use strict and use warnings in every perl script that you write (at least until you know Perl well enough to be able to explain when it is safe or reasonable to not use them). This will save you many many hours of pulling your hair out because of silly typos and coding mistakes.

Regards,
Darren :)

Hi Secode

Your Hash stored format:
$VAR1 = {
          'A' => {
                   'B' => {}
                 }
        };
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This output is now stored..so, using this you can get the key output A
+ B C..so, assign some value for the key if you stored values in hash
$VAR1 = {
          'A' => {
                   'B' => {
                            'C' => 1
                          }
                 }
        };
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#!/usr/bin/perl

use Data::Dumper;

my %line;
$why = "";
$a = A;
$b = B;
$c = C;

$line{$a}{$b}{$c} = 1;
print Dumper(\%line);

$i = 1;

for $a (keys %line) {
for $b (keys %{$line{$a}}) {
for $c (keys %{$line{$a}{$b}}) {

print "$a $b $c\n";

}
}
}
<p>It prints output : A B C
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