# in commons.pl...
package main;
use DBI;  
use Date::Calc qw( Decode_Date_US Add_Delta_Days);
use File::Copy;
# etc.

##</code><code>##

# in main.cgi...
our $q, $variables;
# or, for < 5.6:
#use vars qw/$q $variables/;

BEGIN { require 'commons.pl' }

##</code><code>##

# in commons.pl, _AFTER_ package main LINE!
our $q = new CGI;
our $variables; # etc.