Description:

Hash::MultiKey provides hashes that accept arrayrefs of strings as keys.

Two multi-keys are regarded as being equal if their *contents* are
equal, there is no need to use the same reference to refer to the same
hash entry:

$hmk{['foo', 'bar', 'baz']} = 1;
exists $hmk{['foo', 'bar', 'baz']}; # different arrayref, but true

A given hash can have multi-keys of different lengths:

$hmk{['foo']} = 1; # length 1
$hmk{['foo', 'bar', 'baz']} = 3; # length 3, no problem

In addition, multi-keys cannot be empty:

$hmk{[]} = 1; # ERROR

The next sections document how hash-related operations work in a
multi-key hash. Some parts have been copied from standard documentation,
since everything has standard semantics.