sub ExtractDatabaseFields {
  my $file= shift;
  my @databases= @_;
  my %result;
  foreach my $database (@databases) {
    my @fields= ... #extract data from database
    my $nextfield= .... #find next field
    $result{$database}{next}= $nextfield;
    $result{$database}{fields}= \@fields;
  }
  return \%result;
}

#-------------------
# Accessing

my $result= ExtractDatabaseFields($file,@databases);

#print next available field of database 27:
print $result->{"27"}{next}; 

#extract the fourth field from database 27:
my $field3= $result->{"27"}{fields}[3];

#add another field to database 27:                 
push @{$result->{"27"}{fields}}, "whatever"; 

#loop over the fields of database $x:
foreach $field ( @{$result->{$x}{fields}} ) { .... }