in reply to Newbie: uses/limits of perl in editing files

You can do this with pretty much any scripting / programming language (if it has input/output capabilities and is turing-complete). And Perl may be the most comfortable one for this.

The code to do something like you said could look like

perl -pe '/^REMARK/ and print "the line you want to add\n"' < input_file > output_file

In Section Seekers of Perl Wisdom