while (<DATA>) {
  $_ =~ s/  #substitute
         (?<=\d)  # find position having 1 number preceeding it
         (?=(\d\d\d)+(?!\d)) # ant the position have 1 or more groups of three numbers following it with a non-number at the end
/,/ # put a comma at this position
g;   # do this substitution golbally

  print "$_";
}
__DATA__
123
1234
12345
123446
1234567
1234567.1234