In a hash, there is a one-to-one correspondence between keys and values. If you want to have a one-to-many mapping, you can set the value for a given key to an anonymous array, i.e.

%my_hash = ();
$my_hash{key1} = [];
$my_hash{key1}->[0] = 'value';
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Obviously, you can shorten that up. For for info on arrays, hashes, etc. check out perldata and, for the fancy stuff, perldsc.