full package info contains package manager in square brackets, followed by space separated package list.
[rpm] package1 package2

specifying 'short' as the 3rd parameter omits the package manager type, but does not remove the extra leading space :

$ echo "a$(zabbix_get -s 127.0.0.1 -k system.sw.packages[php.*xml,,short])a"
a php5-xmlreader-5.3.8-4.24.1.i586, php5-xmlwriter-5.3.8-4.24.1.i586a