My take on this would be to put the CGI.pm query object in the 'common code' module too. That way you only have to 'use CGI;' once.

One way:

#!c:/Perl/bin/Perl.exe

use strict;
use warnings;

use MyCommon qw($q check_login common_footer);

my $log_in = check_login();

print $q->start_html(-title => 'log in');
print $q->p($log_in);
common_footer();
print $q->end_html;
[download]

package MyCommon;

use strict;
use Exporter;
use CGI;

our (@ISA, @EXPORT_OK, $q);

@ISA = qw(Exporter);
@EXPORT_OK = qw($q check_login common_footer);

$q = CGI->new;

sub check_login {
  return "login ok";
}

sub common_footer {
  print $q->p('common footer');
}

1;
[download]