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If any of the listed modules are not loaded yet, *base* silently
attempts to "require" them (and silently continues if the "require"
failed). Whether to "require" a base class module is determined by the
absence of a global variable $VERSION in the base package. If $VERSION
is not detected even after loading it, <base> will define $VERSION in
the base package, setting it to the string "-1, set by base.pm".