Like knight, I am also interpolating values from an internal hash. I am dealing with several types of interpolation, using as syntax [FOO] (for simple interpolation) [FOO[x;y]] for a table-based lookup (row x, column y of table FOO), and [FOO(args...)] for user-defined functions. My code looks like

foreach ($string) {
 while (1) {
  s/\[(\w+)\]/&mlookup($1)/e                      and next;
  s/\[(\w+)\[(\d+);(\d+)\]\]/&tlookup($1,$2,$3)/e and next;
  s/\[(\w+)\((\w+(,\w+)*)\)\]/&flookup($1,$2)/e   and next;
  last;
 }
}
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On -some- input, I'm getting a "Use of uninitialized value in substitution iterator" error, although it appears to do the substitution correctly. What does it mean, and how can I get rid of that warning?