If you're familiar with both Ruby and Perl, you'd know that text manipulation is as much an innate part of what Ruby is at it is an innate part of what Perl is.
Perl beats Ruby for such tasks for the following three reasons: people already know Perl, there are more (and better) libraries for Perl, and Perl is somewhat faster on benchmarks. Those are listed in what I believe to be decreasing order of how commonly they come up.
Note that I did not include in the list that Perl is better for text manipulation. That is because I believe it isn't. Ruby can generally express the same concepts as concisely or more concisely than Perl. Ruby has several nice features that Perl does not, for instance I like the fact that Ruby natively lets you iterate over the lines in the string. And Ruby avoids some gotchas that Perl has, for instance using $& somewhere in your program doesn't slow down every other regular expression like it does in Perl.