#!/usr/bin/perl
# your code goes here
{
    my @data;

    $data[1] = 1;       # x1 день

    for (2..4) {
        $data[$_] = 2;  # x2-x4 дня
    }

    for (0, 5..9, 10..19) {
        $data[$_] = 3;  # x5-x0 дней
    }

    sub plural($$$$) {
        return $_[0] < 20
            ? $_[$data[$_[0]]]
            : $_[$data[$_[0] % 10]];
    }
}

for (11, 21, 2, 5, 25, 30, 100501) {
    printf "%d %s\n", $_, plural($_, 'день','дня','дней');
}