Does it somehow auto assign a key/value pair?

Essentially, yes. When you assign directly to a hash it expects a list of key-value pairs. This is why you sometimes see the warning "Odd number of elements in hash assignment" because the list must be even-sized so that each key has a corresponding value. These two assignments are effectively the same:

my %foo = ( baz => 1, frob => 'quux' );
my %bar = ( 'baz', 1, 'frob', 'quux' );
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There's more about this in List value constructors in perldoc.

Does that help?