I'm not clear on everything you're trying to do. But to your main question: how to save the last calculated mean so it can be used after the loop, just make sure you declare the variable outside the loop before it starts, like this:

my $mean;
my $total = 0;
my $count = 0;
for ($j = 2; $j < @tableb; $j++) {
  # do other calcuations
  if(this_line_matches()){
    $mean = $title/$count;  # use $mean from outside the loop
  }
}
print $mean; # now contains the last value calculated inside the loop
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If you don't actually need to calculate the mean for each loop, you could move that calculation to after the loop and only do it once.