sub insertion-sort(@list is rw) {
    for 1 ..^ @list.elems -> $i {
        PRE { [<=] @list[ 0..^$i ] }
        my $value = @list[$i];
        my $j = $i - 1;
        while $j >= 0 && @list[$j] > $value {
            PRE { all(@list[$j..$i]) >= $value }
            @list[ $j + 1 ] = @list[$j];
            --$j;
        }
        @list[ $j + 1 ] = $value;
    }
    @list
}

say insertion-sort( [ 4, 1, 5, 3, 2 ] ).perl;