in reply to Re^2: Searching and Replacing file content within directory
in thread Searching and Replacing file content within directory

sed? I prefer to stick with perl. Here's a 1-liner to search and replace:

perl -i -wpe 's/INPUT_TXT/OUTPUT_TXT/g' file1 [... fileN]

See the perlrun docs for info on using -i.

Use find if needed to generate the list of files, and pipe its output to Perl via xargs.