$ perl -Mstrict -Mwarnings -E ' say join q{,}, reverse map scalar reverse, unpack q{(a3)*}, scalar reverse for qw{ 1 12 123 1234 12345 123456 1234567 12345678 123456789 1234567890 };' 1 12 123 1,234 12,345 123,456 1,234,567 12,345,678 123,456,789 1,234,567,890 $