Are you able to print the contents of @INC on the scripts that are working? Are the scripts that are working and are not working on the same machine?

UPDATE: For example, in your script that is working, right before the use DBD::mysql statement you could insert:

BEGIN {
  open my $fh, ">", "/tmp/name-of-script.INC.txt";
  printf $fh "%s\n", join "\n", @INC;
}
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Also, are the scripts that are working and not working running under the same user?