Add explicit "# key" directive to legacy snippet collection before deprecating filenames-as-triggers

snippets/perl-mode/for

@@ -1,4 +1,5 @@
 #name : for (...) { ... }
+# key: for
 # --
 for (my \$${1:var} = 0; \$$1 < ${2:expression}; \$$1++) {
     ${3:# body...}