I do not know whether this is a correct approach.

$str = "auth_plugin_stack = a,b,EXT::USCC::USCCAuth,c,d";
print "$str \n";
$str =~ s/((\w+::)+\w+,)//;
print "$str \n";
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answer:

auth_plugin_stack = a,b,EXT::USCC::USCCAuth,c,d 
auth_plugin_stack = a,b,c,d
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