my $testNumber = 123456.789;
$testNumber = addComma($testNumber);
print $testNumber;

sub addComma {
	my $number = $_[0];
	$number = reverse $number; 
	$number =~ s/(\d\d\d)(?=\d)(?!\d*\.)/$1,/g; 
	$number = reverse $number;
	return $number;
}