It's still possible but you'll have to use something closer to what sonofason wrote. For each match, you need to create a ($key, $value) pair so the list constructed by map is more like ($key1, $value1, $key2, $value2, $key2, $value3) for the three values. That way, assigning to the hash will insert them as you want.

Now the code you can use to do that, can read for example:

@flat = map { /\w{3}_(\w)/ ? ($1, $_) : () } @inp;
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It'll create a pair like ("A", "abc_A_bla") for a match, and an empty list for no match.

Assign to the hash, and you get:

%inp = map { /\w{3}_(\w)/ ? ($1, $_) : () } @inp;
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