Yet another way to avoid the interpolation problem would be to pre-compile your pattern into a regular expression using qr. You might also consider tightening up your grep condition in case somebody is silly enough to name a directory "confusing.txt" and if you want to exclude symbolic links if on *nix.

...
my $file_mask = qr{\.txt$};
...
my @files =
   grep { m{$file_mask} and -f and not -l }
   readdir DIR;
...
[download]

I hope this is of interest.