Is it a bug that perl produces no warnings in this case?

"." is the concatenation operator, and you used it properly. There's no reason for it to give a warning.

As for a hash element index, it accepts multiple comma-separated values. This was a means of doing multilevel hashes before Perl had true references. These days, one would create a hash or hashes.

$hash{A}{B} = 1;
$hash{C}{D} = 2;
$hash{E}{F} = 3;
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Again, it was used properly, so there's no reason for it give a warning.

Why is it '1c' and not 'f5'?

F5? As in LATIN SMALL LETTER O WITH TILDE? Why do you expect that?

It can be done, though. Just change $; to "\xF5".