use CGI::Simple;
use strict;
use warnings;

my $cgi = CGI::Simple->new;
my $filename = '/usr/local/..../file.txt'; # assumed exists

print $cgi->header(
  -type => 'text/plain',
  -attachment => $filename,
  -content_length => -s($filename),
);

open FILE, $filename or die "Can't open $filename: $!\n";
print while <FILE>;