STOS 2.6 with Compiler version 2.7 doesn't seem to support forward declarations.
That is: the compiler will throw an error if an array is used but declared in a later line number even if the array is initialised error by using a goto\gosub. I don't know about writing compilers, but this seems like quite an advanced thing to do and if it doesn't work in 2.7 it is unlikely it would work in previous versions.
When I used STOS a lot I didn't know what forward declarations were.
Billy Allan discovered a few problems with the compiler when he was writing The Missing Link, but I don't think that was one of them.
Still got, still working: Atari 4Mb STe, 520STFM, 2.5Mb STF.
Hardware: Cumana CSA 354, Ultimate Ripper, Blitz Turbo, Synchro Express II (US and UK Versions).