From the context, I'm assuming your files have two header lines and then one number per line.

use strict;
use warnings;

for (rangeOver(*DATA, 4)) {
    print "$_->[0] to $_->[1] = " .
          ($_->[1] - $_->[0] + 1) . "\n";
}

sub rangeOver {
    my ($fh, $min) = @_;
    my ($start, $end, @ranges, $i);

    <$fh>; <$fh>;
    chomp(@_ = <$fh>);
    
    return [] if $#_ < $min - 1;
    
    $start = $end = shift;
    for (@_) {
        if ($_ == $end + 1) {
            $end++; next;
        }
        push @ranges, [$start, $end]
            if $end >= $start + $min - 1;
        $start = $end = $_;
    }
    push @ranges, [$start, $end]
        if $end >= $start + $min - 1;
    return @ranges;
}

__DATA__
some
header
1
3
4
5
6
8
10
11
12
13
14
[download]