Looks to be a bug with the Perl module you are using.  I have had responses of much larger XML responses than that doing direct calls w/ tools like curl.  Perform a configResolveClass w/ inHierarchical set to true on class "topSystem" on a UCS domain, and you will get the entire XML database returned.  It can be a few MB of data, and I have never had issues getting truncation before.  

 
Here you go.  First you need to login and get yoru cookie via the API with the curl commands below (which assumes HTTP communication):
$ cat aaa.sh 
#!/bin/bash
curl -H "Content-Type: text/xml" -N -s -d "<aaaLogin inName="admin" inPassword="abc123" />" http://1.2.3.4/nuova
$ ./aaa.sh
 <aaaLogin cookie="" response="yes" outCookie="1354316387/74eed11a-35d2-48a1-b569-f75c25ed1482" outRefreshPeriod="600" outPriv="admin,read-only" outDomains="org-root" outChannel="noencssl" outEvtChannel="noencssl" outSessionId="" outVersion="2.1(1a)"> </aaaLogin>
 
Then take the cookie and use it with the following script as illustrated below:
$ cat topsys.sh 
#!/bin/bash
COOKIE=$1
curl -H "Content-Type: text/xml" -N -s -d '<configResolveClass cookie="$COOKIE" classId="topSystem" inHierarchical="true" />' http://1.2.3.4/nuova
 
$ ./topsys.sh "1354316387/74eed11a-35d2-48a1-b569-f75c25ed1482" > topsys.out
As you can see the file I was returned was 1.9 MB:
$ ls -alh topsys.out 
-rw-r--r--  1 ericwill  staff   1.9M Dec  5 12:56 topsys.out