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A hash (sometimes called an associative array) is a "software implemented" associative memory where values are fetched by names or attributes called keys
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A hash is a set of pairs (key, value)
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The entire array is referred to as %dict (for example):
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$dict{key} = value; # NOTE curly braces {} to denote hash
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The values can be used in ordinary arithmetic such as
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$math{pi} = 3.14; $math{pi} += .0016; # sets $math{pi} = 3.1416;
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either pi or 'pi' is allowed for specifying key
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If key pimisspelt is undefined then $math{pimisspelt} returns undef and so one can easily see if a particular key has a value
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Alternatively, the function exists($math{pimisspelt}) returns false unless key pimisspelt has a value
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