#!/usr/bin/perl -w
use strict;
use warnings;

sub format {
        local $_ = reverse sprintf "%.*f",2,shift; # 2=Anzahl Nachkommastellen
        s/(\d{3})(?=\d)(?!\d*\.)/$1,/g;
        tr/.,/,./;
        return scalar reverse;
}
print ":".&format(300000000)."\n";