## Header.pm
package Header;
use strict;
use DBI;
use Exporter;
use vars qw(@ISA @EXPORT);
@ISA = qw(Exporter);
@EXPORT = qw($dbh $userID);
our $dbh = DBI->connect('DBI:mysql:database'....);
our $userID;
1;

## Some_subs.pm
package Some_subs;
use strict;
use Exporter;
use vars qw(@ISA @EXPORT);
@ISA = qw(Exporter);
@EXPORT = qw(get_data());
sub get_data {
	my $sql = "select * from table where id=$Header::$userID";
	return $Header::dbh->selectrow_array($sql);
}
1;

## main ##
use strict;
use Header;
use Some_subs;
$userID = 1;
my $data = &get_data();