I managed to have perl 5.6 on Ubuntu 18.04.1

Thanks for this - just the sort of approach I needed to see !
Nothing wrong with perlbrew's capability to build perl-5.6.2, but this approach makes the procedure clearer, and also saves me having to find out how to influence the Configure flags with perlbrew.
Following those instructions, I built and installed a -Duselongdouble build of perl-5.6.2. (It failed only 2 tests of the test suite.)

BTW, you missed out the tar zxvf perl-5.6.2.tar.gz step, which was enough to befuddle me for longer than I care to admit ;-)
(I recommend that you correct that.)

Cheers,
Rob