You are assigning to $hash{$remote_mt} even when the test /nfs/ fails, and that case $remote_mt is undef. I think you want:

while (<DATA>) {
    unless ( /\A#|^$/ ) {
        if (/nfs/) {
          ($remote_mt,$local_mt) = +(split)[0,1];
          $hash{$remote_mt} = $local_mt;
        }
    }
}
[download]

I would probably write the loop like this:

while (<DATA>) {
  next if /^\s*#/;
  my @f = split(' ', $_);
  if ($f[2] eq "nfs") {
    $hash{$f[0]} = $f[1];
  }
}
[download]

Just a little more robust in case some other field has 'nfs' in it.