my %hash1 = (); # Initialize empty hash. Perhaps unnecessary?
Yes, this is unnecessary. Either: 
my %hash1;

[download]

or: 

my %hash1 = (
    "1", "20",
    "2", "20",
    "4", "19",
    "5", "20",
    "10", "20",
    "6", "18");

[download]

Fat commas are nice too: 1 => 20,

I created a sub "isNull" as a homegrown replacement for "exists". The idea is to check your assumptions (and data) before you operate on that data.
Your isNull sub is not need with your data. Try: 
    unless ($hash2{$thisKey}) {

[download]

instead of: 

    if (isNull(\%hash2, $thisKey)) {

[download]

In reply to Re^2: How best to compare hash values? by toolic
in thread How best to compare hash values? by jimbass

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