Parsing SQL is not trivial, but if these statements are generated by a program, there's a good chance they're consistently simple enough that you can use a regex to do what you want. Assuming your constraint names aren't quoted (so they will match \w), and that the entire clause never breaks over two lines, this should work:

$sql =~ s/
  \bADD\s+CONSTRAINT\s+  # lead-in to clause
  (\w+)\s+               # the constraint name
  \(                     # opening paren of column list
/
  ADD CONSTRAINT $1 FOREIGN KEY (
/x;
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(I haven't tested this code, BTW.) You can adjust this if you have quotes, and so on, but the basic idea is to match around it and then re-write it to what you want.

HTH