Here's a trace of the calls to the script-fu routines, by setting Gimp's TRACE_ALL :

#my $img = eval { Gimp->file_jpeg_load($file, $file) };
file_jpeg_load(
        INT32 run_mode=1        "Interactive, non-interactive"
        STRING filename="test.jpg"      "The name of the file to load"
        STRING raw_filename="test.jpg"  "The name of the file to load"
        ) = (
        IMAGE image=0   "Output image"
        )

#my $drawable = $img->active_drawable;
gimp_image_active_drawable(
        IMAGE image=0   "The image"
        ) = (
        DRAWABLE drawable=2     "The active drawable"
        )

#$img->script_fu_fuzzy_border($drawable, [255, 255, 255], "16", 8, "4"
+, 1, "100", 0, 1);
script_fu_fuzzy_border(
        INT32 run_mode=1        "Interactive, non-interactive"
        IMAGE image=0   "The Image"
        DRAWABLE drawable=2     "The Layer"
        COLOR color=[255,255,255]       "Color"
        STRING value="16"       "Border Size"
        INT32 toggle=8  "Blur Border"
        STRING value="4"        "Granularity (1 is Low)"
        INT32 toggle=1  "Add Shadow"
        STRING value="100"      "Shadow Weight (%)"
        INT32 toggle=0  "Work on Copy"
        INT32 toggle=1  "Flatten Image"
        ) = (
        )

file_jpeg_save(
        INT32 run_mode=1        "Interactive, non-interactive"
        IMAGE image=0   "Input image"
        DRAWABLE drawable=2     "Drawable to save"
        STRING filename="out.jpeg"      "The name of the file to save 
+the image in"
        STRING raw_filename="out.jpeg"  "The name of the file to save 
+the image in"
        FLOAT quality=0.800000  "Quality of saved image (0 <= quality 
+<= 1)"
        FLOAT smoothing=0.000000        "Smoothing factor for saved im
+age (0 <= smoothing <= 1)"
        INT32 optimize=1        "Optimization of entropy encoding para
+meters (0/1)"
        INT32 progressive=1     "Enable progressive jpeg image loading
+ - ignored if not compiled with HAVE_PROGRESSIVE_JPEG (0/1)"
        STRING comment="Kilroy was here"        "Image comment"
        INT32 subsmp=0  "The subsampling option number"
        INT32 baseline=1        "Force creation of a baseline JPEG (no
+n-baseline JPEGs can't be read by all decoders) (0/1)"
        INT32 restart=0 "Frequency of restart markers (in rows, 0 = no
+ restart markers)"
        INT32 dct=2     "DCT algorithm to use (speed/quality tradeoff)
+"
        ) = (
        )
[download]

The relevant Perl lines are inserted above the Gimp trace. I took out the call to flatten-image, since it was already being requested as a parameter to fuzzyborder (and an explicit call didn't change the results either).