You're most of the way there with the likes of File::Find::Rule at your side

use File::Find::Rule;

for(find(file => name => "*.txt", in => "your_dir_here")) {
  ## do stuff here
}
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That will iterate over the filenames of all the files in 'your_dir_here' and its subdirectores with the .txt extension. See. the File::Find::Rule docs for more information on this most marvellous of modules.