Brainfuck [0], Whitespace [1] and INTERCAL [2] come to mind :)

Of course, one could argue none of these are 'serious' programming languages.

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[0]: https://en.wikipedia.org/wiki/Whitespace_(programming_langua...

[1]: https://en.wikipedia.org/wiki/Brainfuck

[2]: https://en.wikipedia.org/wiki/INTERCAL