print add_commas( '+1234567.123456789010'), $/;
print add_commas_arb_string( 'Hello +1234567.123456 Hello -1234567.123456' );

sub add_commas {
    my ( $number ) = @_;
  return undef unless $number;
    ( $number, my $dec ) = $number =~ m!([+-]?\d+)\.?(\d*)!;
  return undef unless $number;
    $number =~ s/(\d)(?=(\d{3})+(\D|$))/$1,/g if length($number) > 3;
  return $dec ? "$number.$dec" : $number;
}

sub add_commas_arb_string {
   my ( $string ) = @_;
   $string =~ s/([+-]?\d(?:\d*\.\d+|\d*))/add_commas($1)/ge;
  return $string;
}

__DATA__
+1,234,567.123456789010
Hello +1,234,567.123456 Hello -1,234,567.123456