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

##</code><code>##

195,000
42,000
2,100