Date::Manip is designed to handle multiple values for month abbreviations -- if you ask the maintainer, s/he might well be willing to modify it to include "sep" for French.

In the meantime, it would be easy to modify a local copy .

sub Date_Init_French { # [...]
$$d{"month_abb"}=
    [["jan","fev","mar","avr","mai","juin",
      "juil","aout","sept","oct","nov","dec"],
     ["jan","f${e}v","mar","avr","mai","juin",
      "juil","ao${u}t","sept","oct","nov","d${e}c"],
     ["jan","fev","mar","avr","mai","juin",
      "juil","aout","sep","oct","nov","dec"]];
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