$ perl -Mstrict -Mwarnings -e '
    my @input = qw{12 14 15 20 21};
    my $ranges = 3;
    my @bins;

    for (@input) {
        ++$bins[$_ / 20];
    }

    for (0 .. $ranges - 1) {
        printf "%d-%d %d\n" => $_ * 20 || 1, $_ * 20 + 19, $bins[$_] // 0;
    }
'
1-19 3
20-39 2
40-59 0