$ perl -Mstrict -Mwarnings -e '
    use List::Util qw{first};

    my @input = qw{12 14 15 20 21};
    my @range_ends = qw{19 29 39};
    my (@bins, %range_index_cache);

    sub get_range_index {
        ( first { $_[0] <= $range_ends[$_] } 0 .. $#range_ends ) // @range_ends;
    }

    for (@input) {
        ++$bins[$range_index_cache{$_} //= get_range_index($_)];
    }

    for (0 .. $#range_ends) {
        printf "%d-%d %d\n" =>
            $_ ? $range_ends[$_ - 1] + 1 : 1, $range_ends[$_], $bins[$_] // 0;
    }
'
1-19 3
20-29 2
30-39 0

##</code><code>##

$ 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