dog:cat:pig       <== skip, only has three fields
Amy:Ann           <== skip, only has two fields
ape:ant:bug:car   <== match this line (it has four fields)
                      but we only need to keep 'ape' and 'ant'

##</code><code>##

while (<DATA>) {
    chomp;
    (my ($first, $second) = split /:/) == 4 or next;
    print "$first $second\n";
}

__DATA__
dog:cat:pig
Amy:Ann
ape:ant:bug:car