When you run your system, a subshell is created, the export is run, and the subshell immediately exits. Thus, the value is not changed in the scope of the running Perl script. If you want to change the environment variable's value at the level of the Perl script, you can use %ENV:

$ENV{'CLASSPATH'} = "$ENV{'CLASSPATH'}:/usr/users/private";
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Any Java that you then invoke from Perl using system or backticks will have the new CLASSPATH. Note that when the Perl script exits, the CLASSPATH will remain at its original value, not the value set within the Perl script.