use Socket;
my @ip = qw( 10.0.4.100 10.0.4.99 10.3.5.21 10.0.4.101 10.15.21.6);

@ip = do {
    my $prev;
    map { defined($prev) && $prev == $_ ? () : ( $prev = $_ ) }
      sort { $a <=> $b }
      map { unpack "N", inet_aton $_ } @ip;
};
for my $i ( 0 .. $#ip ) {
    if (!$i || $i == $#ip ||
         $ip[$i-1] != $ip[$i] - 1 ||
         $ip[$i+1] != $ip[$i] + 1 )
    {
        print $ip[$i-1] == $ip[$i] - 1 ? "-" : " " if $i;
        print inet_ntoa( pack "N", $ip[$i] );
    }
}
print "\n";
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