I am not sure if this is documented as I open the file in read-mode

From open (emphasis mine):

As a special case the three-argument form with a read/write mode and the third argument being undef:

open(my $tmp, "+>", undef) or die ...
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opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which makes +> the only useful and sensible mode to use.)