Is there an explanation, why split() behaves like this:

Yes, because its documented that split behaves that way, split splits strings apart into pieces (split cuts), even if there is nothing in between

Read perldoc -f split and consider this

use Data::Dump qw/ dd /;
dd( split /\D/, q/12Q34/ );
dd( split /\D/, q/12ab34/ );
dd( split /(\D)/, q/12ab34/ );
__END__
(12, 34)
(12, "", 34)
(12, "a", "", "b", 34)
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Q is not a digit between 12 and 34
empty string "" is not a digit between a and b
empty string "" is not a digit between a and b (a and b are preserved not discarded

split cuts a string apart, discarding the cut pieces unless you (keep) them