my @currenttime=(); ## or initialize to the start time
my @totaltime=(); ## will hold the sums for each ID
foreach (@databaseline) {
## realistically, this will be something like 
## while (@values = $sth->fetchrow) when you convert to
## using SQL ( Look at the DBI module and the corresponding
## DBD module for your database )
  my ($dbtime,$code,$id)= &parseline($_); 
## This routine would split the line and convert 
## the date, passing back the useful values.
## When you get it out of SQL, you should have an array
## of values anyway, so it'll only be necessary to
## convert the date.
  $totaltime[$id]+=($dbtime-$currenttime[$id]) if $code eq 'open';
  $currenttime[$id]=$dbtime;
## we move the baseline to the time of the current record
}
## now the @totaltime array has the values you want.