I don't know about Perl, but if you want to get up and running with R for some basic data analysis, I'd say:

1 week to get used to the syntax and do basic data manipulation such as contructing, editing data frames, reading and writing files etc. You can also get going with the simple graphing functions quite quickly.

1 week to learn the statistical stuff - scatter plotting, correlations, linear regression modelling. Of course, I'm talking about just learning the basic syntax. Doing regression properly could take a long time to learn.

The R Book by Crawley is quite a good introduction, especially if you want to brush up on some basic statistics. It's not very in-depth though, so you might have to move on to more advanced texts after a month or so.

I've used R since 1.4 ~ (2001). Compared to other dynamic languages, I think it's harder than most. Why? Because there's a lot more semantics. There might be R one order of magnitude more keywords for R than for Perl, Ruby or Python (PHP doesn't count :) ). There are two object systems, none of them very intuitive. You need to learn functional programming. 1600 packages and counting means more functions and deep concepts to understand. And last, you need to understand the math of whatvever it is you are using. Forget doing point-and-click, black-box analyses on SPSS.

Downsides include poor programming tools/debugger and pass-by-value. Still, it's one of my favorite languages.