one thing you can do for each line is just substitute the start of the line for the current number. as a one liner though:

perl -pi.bak -e '$count++;s/^/$count. /' file_name
[download]

or in your could you could change:

foreach my $line (@file) {
  (undef) = <FILE>; # stores number of lines into $.
  $count++;
  print FILE, "$count. ";
  if ($count == $.) {last; }
}
[download]

to something like:

foreach my $line (@file) {
    $count++;
    print FILE, "$count. $line";
}
[download]

or:

foreach (@file) {
    $count++;
    s/^/$count. /;
    print FILE;
}
[download]

As always TMTOWTDI. I am curious as to why this line:

(undef) = <FILE>; # stores number of lines into $.
[download]

and this line:

if ($count == $.) {last; }
[download]

is necessary.

-enlil