while ($number =~ s/(.*\d)(\d\d\d)/$1,$2/g) { }

Beginning with Perl version 5.5, we can compile regular expressions and keep references to them without using a match or substitution operator. We can do all of our regular expression handling and preparation before we actually want to use them. Since a regular expression reference is just a scalar like any other reference, we can store it in an array or hash, pass it as an argument, interpolate it into a string, or use it in the many other ways we can use a scalar.