Oops, scratch that first one.. I did say I was a newbie right?

 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 $hidden_string eq "true"{
           $hidden_string = "";
           &dothis;
        }
[download]

Originally posted as a Categorized Answer.