Coming up with the "best" solution depends a lot on variables like how large the files are, what kind of performance you need, and how you'll come up with the new file name. However, here's the simplest way of solving that (if memory usage and time are no issues).

use strict;
local $/ = undef; # grab everything from file

open FILE, "my_file" or die $!;
foreach $data_block (split /match_instance/, <FILE>) {
    open OUTPUT, "new_file_name" or next;
    print OUTPUT $data_block;
    close OUTPUT;
}
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Note that whatever string we look for ("match_instance" in this example) will get deleted by the nature of split. You can enclose match_instance in parenthesis if you want it included. But then you'll end up with some array that looks like "match_instance", "data", "match_instance", "more data", etc. So you couldn't use a foreach to process it.

-Ted