I finally worked out my solution. The problem was to consolidate all the constants that must be modified when the code is installed somewhere, into a single package. Accessible from all other packages that make up the project.

package Gbl;
# simple holds Global data loaded from the config file eventually
use strict;
require  Exporter;
BEGIN{

}

my @ISA = qw(Exporter);
my @EXPORT = qw();
my @EXPORT_OK = qw();

use vars qw(
  $EmailAddr
  $VERSION
);
$VERSION = sprintf "%d.%03d", q$Revision: 1.0 $ =~ /: (\d+)\.(\d+)/;
$EmailAddr = "a\@b.com";

1;
__END__
[download]

So now all uses of the email address, in any package (or hunk of code) are

$From = "$Gbl::EmailAddr";
....or somewhere else ...
if ($Sender =~  /$Gbl::EmailAddr/) { ...
[download]

This also allows me to find all direct uses of this configurable data by `grep 'Gbl::' *`.

Thanks all for the help.