Your Hash stored format:
$VAR1 = {
          'A' => {
                   'B' => {}
                 }
        };

##</code><code>##

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
                          }
                 }
        };

##</code><code>##

#!/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