The hash is being defined in this line:

$m{$_} = ++$i for qw[Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec];
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After having executed this statement (or maybe I should say this loop), the following structure exists in memory:

( Jan => 1, Feb => 2, ...)
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Please beware that this code is fairly sloppy, with no declaration of variables, it is not an example to be followed.