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"]];
