And yet another way, with the sub straight from The Perl Cookbook...

while (<DATA>) {
    chomp;
    print commify($_), "\n";
}


# From _The Perl Cookbook_, recipe 2.17

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

__DATA__
195000
42000
2100
[download]

This outputs

195,000
42,000
2,100
[download]

Here's the discussion in the book:

It's a lot easier in regular expressions to work from the front than from the back. With this in mind, we reverse the string and make a minor change to the algorithm that repeatedly inserts commas three digits from the end. When all insertions are done, we reverse the final string and return it. Because reverse is sensitive to its implicit return context, we force it to scalar context.