Interesting homework problem. However, I'm in a generous mood. Next time, actually think before you post.

my @list;
my ($first, $last) = (undef, undef);
NUM:
foreach my $num (@read) {
    unless (defined $first) {
        $first = $last = $num;
        next NUM;
    }

    if ($num > ($last + 1)) {
        push @list, ($last > $first ? "$first-$last" : $first);
        $first = $last = $num;
        next NUM;
    }

    $last = $num;
}

push @list, ($last > $first ? "$first-$last" : $first)
    if defined $first;
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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.