That fails on
/* outer /* mid */ /* mid */ outer */
Try:

($re = $_)=~s/((\Q$start\E)|(\Q$end\E)|.)/${['(','']}[!$2]\Q$1\E${[')'
+,'']}[!$3]/gs;
$re = join'|',map quotemeta,eval{/$re/};
warn $@ if $@ =~ /unmatched/;
print join"\n",/($re)/g,"";
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