Fabricating hash keys

Question

I have inexplicable behavior in which Perl is fabricating a "null" hash key out of thin air. I'm simply grabbing keys from an existing hash and creating a new hash with the same keys. Despite there being only 15 keys in the original hash, the newly formed one has gained a mysterious 16th key.

Here is the relevant code:

DO_SOMETHING(\%Hash1, \%Hash2);

sub DO_SOMETHING {
    my %Hash3 = undef;
    my $count = 0;                                       # reset counter
    my ($Hash1Ref, $Hash2Ref) = @_;                      # grab arguments to subroutine
    my %Hash1 = %$Hash1Ref;                              # dereference hash #1
    my %Hash2 = %$Hash2Ref;                              # dereference hash #1
    my @Hash2keys = sort {$a <=> $b} keys %Hash2;        # grab keys from Hash #2
    print "Hash #2 keys = ", join (',', @Hash2keys), "\n"; # print keys in Hash # 2
    my $NumHash2keys = @Hash2keys;
    print "Number of Hash #2 keys = $NumHash2keys\n";    # print number of keys in Hash #2
    print "Building new hash (Hash #3) using keys from Hash #2\n";
    foreach my $Hash2key (@Hash2keys) {                  # iterate through Hash # 2 keys
      $count ++;                                         # count number of iterations
      $Hash3{$Hash2key} = 1                              # mirror keys from Hash #2 in a new hash (Hash #3)
    }   
  
    print "Number of Hash #2 key iterations = $count\n"; # confirm number of iterations through Hash #2 keys
    my @Hash3keys = keys %Hash3;
    print "Hash #3 keys = ", join (',', @Hash3keys), "\n";
    my $NumHash3keys = @Hash3keys;
    print "Number of Hash #3 keys = $NumHash3keys\n";
}

And here is the output:

Hash #2 keys = 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
Number of Hash #2 keys = 15
Building new hash (Hash #3) using keys from Hash #2
Number of Hash #2 key iterations = 15
Hash #3 keys = 1,10,4,12,3,13,6,2,15,14,7,,11,9,8,5
Number of Hash #3 keys = 16

I can't for the life of me figure out where the 16th "null" key is coming from. It appears to be assigned during an invisible loop.

Answer 1

Don't initialize %Hash3 with undef.

Change:

my %Hash3 = undef;

to:

my %Hash3;

If you add the following line to the top of your code (which you should always do), you will get a warning message:

use warnings;