An accountant writes:
Much the same way as one learns anything else. Read, practice, discuss. The thing Perl has above any other language in my experience is its community. I don't know (& can't be bothered to Google) if there's a Perl Mongers group in Athens, but if there isn't, try founding one. You may not be able to help your fellow mongers technically, but if you put some work into the organisation, it will be clear that you are not a freeloader and will get lots of help. At least, that was my experience in another country in the days before Perl was invented.
You know how to research things and evaluate sources (or bloody well ought to if you have or are working to a PhD - yes, I have one of those too). Apply the same techniques to computing sources. You found this site. What do you think of it? My evaluation is that it's very good on technical matters, pretty amusing when it tries to be (polls & some of the CB, for example) and occasionally allows itself to become a forum for flame wars. I try to learn from the first, laugh at the second and ignore (possibly downvoting) the third. But it might not suit you. There are other sites, some language specific, some (Stack Overflow is possibly the biggest) polyglot.
Then write. You have your BioPerl needs for your PhD, but if that stops providing you with a cause to write code, find something else that interests you and write code for it. Write documentation, too. It's fine to document someone else's work. I am convinced that no-one (certainly including me) documents their own code well. So take some of the code you've had trouble understanding and write better docs for it and publish them. If you want something to start on where you can be certain you won't be flamed, my one CPAN module is http://search.cpan.org/~davies/XML-Lenient/lib/XML/Lenient.pm, and you can be as brutal about those docs as you like without causing offence.
As well as writing, talk. Attend things like the London Perl Workshop (London Perl Workshop 2016). When you feel you have something to say, say it, even if that's just giving a 5 minute "lightning" talk.
Read and listen to the feedback you get from these activities, but with your PhD evaluator switched on. Some of the feedback will be nonsense and some of it personal taste.
As someone said, I think quoted in Peters & Waterman's "In search of excellence", "the first hundred years are the hardest".
I can't end without a reference to "teach yourself programming in 10 years": http://norvig.com/21-days.html.
In reply to Re: How does one learn perl programming efficiently - if they do not come from computer science background?