To hippo and tybalt89:

Thank you both for participating in this stimulating dialogue. There are always new things to learn about Perl regexes, and as ikegami said, this is an exercise in nested s///, which I very much want to master. Here are nine more tests I added to hippo's test paradigm, to wit:

sub getArray {
    my $n = $_[0];
    my @examples = (
        "The population of the U.S. is ${\($n*340389145)} people",
        "The frequency is ${\($n*13824)}Hz",
        "This is a real number, ${\($n*723540.34034)}",
        "${\($n*1045.357)}",
        "${\($n*834569.334656)}",
        "${\($n*1000000)} B.C.",
        "My daily compensation is \$${\($n*2234.36)} per hour.",
        "${\($n*2349134)}",
        "${\($n*123456)}",
    );
    return @examples;
}

my $n = 4;
my @have = getArray(1);
my @want = getArray($n);

for my $i (0 .. $#have) {
    my $have = $have[$i];
#   say "$have";
    my $want = $want[$i];
#   say "$want";
    (my $work1 = (($have) =~ s{ \d+\.?\d* }{ $& * $n }erx )) =~ s{ \d+
+ }{
       $& =~ s/ (?<=\d) (?= (?:\d{3} )+ (?!\d) ) /,/xrg; }ex;

    (my $work2 = $want) =~ s{ (?<int> (\d+)) }{
        $+{int} =~ s/ (?<=\d) (?= (?:\d{3} )+ (?!\d) ) /,/xrg; }ex;

    is $work1, $work2, 'Factor and insert commas where appropriate';
}
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As I said, I could not make tybalt89's commify variation work, but perhaps he can. As you can see from getArray(), I made the tests more robust. I agree with hippo re performance. If I were going to include a construction as complicated as this that gets called many times, I'd Benchmark it to do comparative analysis.

Regards