1  perl -pi -w -e 's/search/replace/g;' *.txt

Bear in mind what's between the single quotes after the -e option is a line of Perl code and the search argument is a regular expression, so some characters will have special meanings. You can escape special characters (like the forward-slash, dot, caret, dollar sign, and so on) by preceeding them with a back-slash.

Find files that contain a text string:

   1  grep -lir "some text" *

The -l switch outputs only the names of files in which the text occurs (instead of each line containing the text), the -i switch ignores the case, and the -r descends into subdirectories.