At least the envelope on that one makes more sense.

I get a decoding of the first 8 bites plus envelope of:

0x4750 v=0 f=00000101 srsid=27700
envelope#2 -> 6 elements, mode=little-endian => 'd<6'
339881.51, 340581.897, 442566.608999999, 443257.832, 0, 0
[download]

And the minx,miny are smaller than maxx,maxy -- so whether that's a good thing or not, I don't know.

The main takeaways of my input is that you need to use the < for litte-endian and > for big-endian, and that way you won't be guessing in the future whether your system and/or perl have native big- or little-endianness. (Also, even if you'll always be on the same processor, having those directions explicit in the code will make future development/maintenance so much easier.)