As a start have a look at this:

use strict;
use warnings;

my %conns;
while(<DATA>){
        $conns{$1} = $2 if /(\d+)-(\d+)/ and $2 > ($conns{$1}//0);
}
printf "%d:\t%d\n", $_, $conns{$_} for sort keys %conns;

__DATA__
4-12
7-10
7-12
8-13
[download]

Basically, for each start of interval you store the biggest end of interval. It is not clear to me whether you want to check for other, more comprehensive types of overlaps.