Hash key value changing to array reference after subroutine in perl - perl

Question

I'm creating keys outside and inside a subroutine on the same hash. However, after the subroutine, the values in the keys I created before the subroutine is called, are now interpreted as array references.
#!/usr/bin/perl
use module;
use strict;
use warnings;
my %hash;
my $count = 0;
my #array = ("a", "b", "c", "d");
for my $letter (#array) {
$hash{$letter} = $count;
$count++;
}
# need "\" to pass in hash otherwise changes
# will get lost outside of subroutine
foreach my $x (sort keys %hash) {
print "first $hash{$x}\n";
}
module::add_ten(\%hash);
foreach my $p (sort keys %hash) {
# $hash{$p} is printing array references, but before it was
# printing the value I desired. What did the subroutine do?
print "second $hash{$p} $hash{$p}->{ten}\n";
}
and here is the module with the subroutine
package module;
sub add_ten {
my $count = 10;
# this passes the full array as reference
my ($hash_ref) = #_; # $hash_ref is actually %hash (yes, the % is not a typo)
my #keys = keys $hash_ref;
foreach my $ltr (sort keys $hash_ref) {
$hash_ref->{$ltr} = { ten => $count };
$count++;
}
}
1;
here is the output:
first 0
first 1
first 2
first 3
second HASH(0x7ff0c3049c50) 10
second HASH(0x7ff0c3049bc0) 11
second HASH(0x7ff0c3049b90) 12
second HASH(0x7ff0c3049b60) 13
I'm expecting the output to be:
first 0
first 1
first 2
first 3
second 0 10
second 1 11
second 2 12
second 3 13
I modified my module:
package module;
sub add_ten {
my $count = 10;
# this passes the full array as reference
my ($hash_ref) = #_; # $hash_ref is actually %hash (yes, the % is not a typo)
my #keys = keys $hash_ref;
foreach my $ltr (sort keys $hash_ref) {
$hash_ref->{$ltr}{ten}=$count;
$count++;
}
}
1;
and the main script (needed to comment out use strict to get it to work):
#!/usr/bin/perl
use module;
#use strict;
use warnings;
my %hash;
my $count = 0;
my #array = ("a", "b", "c", "d");
for my $letter (#array) {
$hash{$letter} = $count;
$count++;
}
# need "\" to pass in hash otherwise changes
# will get lost outside of subroutine
foreach my $x (sort keys %hash) {
print "first $hash{$x}\n";
}
module::add_ten(\%hash);
foreach my $p (sort keys %hash) {
print "second $hash{$p} $hash{$p}{ten}\n";
}
But this is what I was trying to get to.

Answer 1

$hash_ref is a reference to %hash, so when you change the values of the elements of the hash referenced by $hash_ref, you're changing the values of the hash %hash.
That means that when you do
$hash_ref->{$ltr} = { ten => $count };
You are doing
$hash{a} = { ten => 10 };
It should be no surprise that $hash{a} no longer contains zero. You'll have to change your data structure. You could use the following:
$hash{a}{value} = 0;
$hash{a}{subhash}{ten} = 10;