one way is like this:

#!/usr/bin/perl -w

use strict;
my $file = "/home/trix/lines";
my $line;
my @array;

open (FILE, "$file") || die "Can't open $file: $!\n";

while ($line = <FILE>) {
chomp $line;
next unless ($line =~ /Down/);
my ($downtime, $down) = split(' ', $line);
push (@array, $downtime);

}
my $agg = eval join '+', @array;
 
  #the above beauty came from somewhere here; don't remember where

print "My downtime is $agg\n";
[download]

....if I am understanding correctly that the first element in each log file is actually a number, and there is only one other element, the interesting one being "Down".