This a quick demonstration of what is going on under the Perl debugger:

  DB<1>  @c = ('jan', 1, 'feb', 2, 'mar', 3);

  DB<2> x \@c
0  ARRAY(0x80359de8)
   0  'jan'
   1  1
   2  'feb'
   3  2
   4  'mar'
   5  3
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So, @c is an array.

  DB<3> %d = @c;

  DB<4> x \%d
0  HASH(0x8042f6d8)
   'feb' => 2
   'jan' => 1
   'mar' => 3
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Copying @c into %d sorts of promote the data structure into a hash (provided the number of elements is even). This is what happens in your code snippet. The @_ array contains just a flattened list. $header take the first element of @_, and the rest of @_ is copied into the %types hash.