When you use split, the pattern must match what separates what you want. In this case, the separator is the empty string between a character at an odd positions and a character at an even position. That's not exactly straightforward to match, but it's possible.

$ perl -E'say for split /(?!^|\z)(?(?{ pos()%2 })(?!))/, "0102030405"'
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[download]

Here, it's simpler just to match what you want grab rather than what separates them, so just use a m//g:

$ perl -E'say for "0102030405" =~ /(..?)/sg'
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