Perl is a good language for isolating and extracting data from formatted text (i.e. (X)HTML). Some parts of Perl are like C++, most of it isn't; but it's easy to grasp.

XSLT can be used to extract and present data from well-formed XML (i.e. XHTML), but it is very 'strict' with regard to input, and somewhat constrained with regard to output.

Sieving data, as you put it, isn't really webdesign; but both of those languages can be used to generate pages and manage websites aswell.. It depends to a great extent on what you're trying to do overall.

i would agree with pearl, i believe it should be in every admins toolkit.
as a side note, as you allready know C/C++, you might want to try using system(); (man 2 system) to call external programs such as sed and awk, the text manipulation programs