You might want to reconsider the substitutions that operate on $full_path. As they are now, ANY occurance of 'sub' or 'srt' in $full_path will be replaced with 'txt', not just the file extension (which I assume you are trying to do). If you want to replace only the extension, you could do something like:

$full_path =~ s/sub$/txt/i;
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Alternatively, you could use the File::Basename and/or File::Spec modules to perform the path/filename operations for you (i.e., split off the suffix and create a path/filename for the new file).