$ ack MY_FORMAT *
Goner2.c
170:        printf(MY_FORMAT, sqrt(2.0));

Goner2.xs
14:        printf(MY_FORMAT, sqrt(2.0));

Makefile
18:#     DEFINE => q["-DMY_FORMAT=\"%.16e\""]
165:DEFINE = "-DMY_FORMAT=\"%.16e\""
315:    PASTHRU_DEFINE="\"-DMY_FORMAT=\\\"%.16e\\\"\" $(PASTHRU_DEFINE)"\

Makefile.PL
26:    DEFINE => q{"-DMY_FORMAT=\"%.16e\""},

$ perl -Mblib -MGoner2 -e" Goner2::beGone() "
1.4142135623730951e+000