# today's data -- your starting point
my %SHIPS = (
  SHIP1 => [
    {datetime => '20080318-0730', latitude => '10.33', longitude => '-05.45'},
  ],
  # other ships
);

##</code><code>##

use Data::Dumper;
print Data::Dumper::Dumper([\%SHIPS],[*SHIPS]); # save to file, don't print

##</code><code>##

my %SHIPS;
do 'dumped_file';

##</code><code>##

while (<$CSV>) {
  chomp;
  # assume csv is SHIP1,DATETIME,LATITUDE,LONGITUDE.
  # modify accordingly..
  my ($shipname, $datetime, $latitude, $longitude) = split(/\,/$_);
  my %today_details = (datetime => $datetime, latitude => $latitude, longitude => $longitude); # this is what will get pushed on the arrayref
  my @arr = @{$SHIPS{$shipname} || []}; # get curr contents
  push (@arr, \%today_details); # push today's data
  $SHIPS{$shipname} = \@arr; # push back
}
# that's you done with the merge