sub mail {
  my ($uid) = @_;                  #  <-- assign argument
  print "uid in sub=$uid\n";
}
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it maybe worked before because you also filescoped a $uid, but after passing it to the routine as argument you should also assign it.

EDIT: At least that may be the reason why strict didn't complain. Anyway in many aspects your code is too weird for me to try it out.