###This takes counts the keys from the the in the hash previously populate from the file and puts it into scalar
my $oldsize = keys %old;
#Does the compare, and if the count of hash keys from the <$in> and different from the hash keys of @command has increased (i.e. new keys added)
if( keys %old > $oldsize ) # only write if new ips added
## If file is there then read contents into keys and associating a value of blank or () means new line?
## why wouldn't you do @old{<$in>} = undef; , would it be wrong?
@old{ <$in> } = (); # note: keys have \n
In this case no difference but possibly misleading you to think
@old{<$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
};
poj
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