my $count = -1;  # set to -1 for loops sake and autoincrementing
my $placeHolder = 0;  # remembers place in array for the first down
my $totalDownTime = 0;

while ( $count++ <= $#lines )
{
    if ( $lines[$count] =~ /Down/ )  # finds first down;
    {
        $placeHolder = $count;
        # the following while increments until it finds the next up,
        # or the end of the array
        while ( ( $count++ <= $#lines ) && ( $lines[$count] =~ /Down/ ) )
        {
            # do nothing in the while loop
        }
        $totalDownTime += sum( $lines[$placeHolder], $lines[$count] );
        # sum will be a function defined by you that takes the lines with
        # down and its corresponding up, parses them,
        # and returns the time difference
    }
}