just use this.getPackageInfo(pname,f); in function(pname,f){...} instead of (pname,f)=>{...}, so finally:

Java.perform(()=>{
    const jPM=Java.use('android.app.ApplicationPackageManager');
    jPM.getPackageInfo.overload('java.lang.String','int').implementation=function (pname,f) {
        console.log("Called => getPackageInfo (Flag="+f+", Pkg="+pname+")");
        return this.getPackageInfo(pname,f); 
    }
});