If it is really true that you '... cannot change the fact that [you are] given the actual substitution regex, complete with delimiters, in a scalar variable...', then:

>perl -wMstrict -le
"$_ = 'this is a bad idea';
 for my $substitution (@ARGV) {
   print qq{substitution: $substitution};
   my $subs = eval $substitution;
   if ($@ or not defined $subs) {
     die qq{$substitution failed: $@};
     }
   print;
   }
"
s/this/that/  "s/is/would be/"  "s/(a)???/$1 very/"
substitution: s/this/that/
that is a bad idea
substitution: s/is/would be/
that would be a bad idea
substitution: s/(a)???/$1 very/
s/(a)???/$1 very/ failed:
  Nested quantifiers in regex;
  marked by <-- HERE in m/(a)??? <-- HERE / at (eval 3) ...
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