###This takes counts the keys from the the in the hash previously populate from the file and puts it into scalar

#Does the compare, and if the count of hash keys from the <$in> and different from the hash keys of @command has increased (i.e. new keys added)

## If file is there then read contents into keys and associating a value of blank or () means new line?
## why wouldn't you do @old{<$in>} = undef; , would it be wrong?

In this case no difference but possibly misleading you to think @old{<$in>} = 1; would assign the value 1 to all the keys from <$in> which is not the case. Try this simple example

#!/usr/bin/perl
use strict;
use Data::Dumper;

my @keys = ('a','b','c');
my %hash=();
@hash{@keys} = 1;
print Dumper \%hash;
__END__
$VAR1 = {
          'c' => undef,
          'a' => 1,
          'b' => undef
        };
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