I recently coded something very trivial in Perl. What surprised me was how long it took me.
It was a classic Perl situation; rows of text in one format that needed to turn into rows of text in another format. The classic Perl solution here is a foreach, a regex, and a sprintf() call. I've been coding Ruby so much that my first step in coding this was to try to build objects, and then an iterator.
Objects in Perl exist, in a sense, but they're nothing like Ruby objects. The brilliant book Higher-Order Perl actually shows you how to build an iterator in Perl, but that's for unusual circumstances. You don't write Perl that way. It's backwards; it disregards the relevant idioms and adds extra steps. It makes a molehill into a mountain. My solution was all wrong because I was trying to write Ruby in Perl.
It's kind of like the difference between being fluent in a foreign language and simply going native. On the one hand it's great that Ruby's style has become, to some extent, a matter of instinct for me. But I was surprised that it took me as long as it did to mentally switch gears.
When I first started doing Ruby, getting away from Java seemed almost heavenly to me. But now I almost want to code something in Java again to see how it feels different. When you learn new languages, you want to expand your skill set, not replace it.