my $zip = $zip_server->get_zip_codes();
print "selectbox-header";
for (@$zip) {
   print "selectbox line";
}
print "selectbox-end";

##</code><code>##

...in common initialisation code...
our $zip;
$zip=$zip_server->get_zip_codes();

...where the zip-code is needed...
print "selectbox-header";
for (@$zip) {
   print "selectbox line";
}
print "selectbox-end"

##</code><code>##

...in the common initialisation code...
our $zip;
sub zip_init {
  $zip = $zip_server->get_zip_codes() unless $zip;
}

...at every use...
zip_init();
print "selectbox-header";
for (@$zip) {
   print "selectbox line";
}
print "selectbox-end";

##</code><code>##

...in initialisation section...
our $zip;
tie $zip .... # magic here
sub ZIP::TIE::FETCH {
   # smoke and mirrors here
   $zip = $zip_server->get_zip_codes();
   untie $zip; # and leave the data in $zip
}

..and where we use it...
print "selectbox-header";
for (@$zip) {
   print "selectbox line";
}
print "selectbox-end";