Why are some of my ranges insane?

I tried parsing a common string depiction of ranges (e.g. 1-9) into actual ranges (e.g. 1 .. 9), but often got weird results when including two digit numbers. For example, 1-10 results in the single value 1 instead of a list of ten values and 11-20 gave me four values (11 10 21 20), half of which aren't even in the expected numerical range:

put get_range_for('1-9');
put get_range_for('1-10');
put get_range_for('11-20');

sub get_range_for ( $string ) {

    my ($start, $stop) = $string.split('-');

    my @values = ($start .. $stop).flat;

    return @values;
}

This prints:

1 2 3 4 5 6 7 8 9
1
11 10 21 20

Instead of the expected:

1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20

(I figured this out before posting this question, so I have answered below. Feel free to add your own answer if you'd like to elaborate).