Its right you do not have to open file twice. If the files are small... you can always place file contents in and array and use them.

open(FILE, "< nanotest.txt") || "can't open $file: $!";
my @arr = <FILE>;
my $last = $#arr+1; ## you can also use $. instead of $#arr
my $count;
foreach (@arr) 
{
    $count++;
    if ($count == $last) 
    {
        # Validate last line
    }
}
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