in the grep, you have a regex, with a rather strange looking pattern.

my @files = grep {/_*_.txt/}  readdir DIR;
[download]

perhaps:

my @files = grep {/\.txt$/}  readdir DIR;
[download]

is all that you need? The $ anchors the regex to the end of the string. An underscore "_" would be unusual before a .txt ending. Without escaping the "." like "\.", the dot means any character, which doesn't look like that you want.

When debugging, print @files to make sure that you are getting the files that you want.