I'm still a newbie, but I ran into this yesterday. I wanted to be able to return from a sub that required a variable to be set

 if $hidden_string eq "true"{
           &dothis;
        }
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To avoid all the problems with hitting the back button, I changed it to this

 if (defined($hidden_string)){
           $hidden_string = "";
           &dothis;
        }
[download]

Originally posted as a Categorized Answer.