open(FILE, "< $file") or die "can't open $file: $!";
$count++ while <FILE>;
# $count now holds the number of lines read

open(FILE, "< $file") or die "can't open $file: $!";
for ($i=0; <FILE>; $i++) { 
     if ($i == $count) {
        # Validate last line
     }
}