Akira, that kind of questions can't be answered on the abstract. There's no universal less than operator that applies to dynamic languages, and I know a lot of Perl fellows that know Python and Ruby, no contradiction, if you restrict yourself to language X then it is a matter of time you become blind and biased.
I am more or less active in the Perl communtity, but my last pet project is being written in Python. Why? Because among other technical requirements I need something like subprocess. Fine, go with Python then.
Oh, and remember the differences are by design not by age. Perl and Python have both about 18 years. Ruby has about 12 years.
If you follow the evolution of Perl 6 you'll see the fundamentals of Perl's design are there because they are a choice. Of course there's nothing on earth that pleases everyone, that's why we have different languages. I know people who can't stand Python whitespace conventions, I know people who think Ruby ends up being too dense.
In the end you have preferences based on a mixture of technical needs, personal taste (which weights a lot and is subjective), and then practical stuff such as the languages your team actually know, etc.