The variable $_linuxprefix is not supposed to be expanded within single quotes.
The variable $_linuxprefix is not supposed to be expanded within single quotes.
The BACKUP may not be functioning properly.
It should be wrapped in double quotations.
Run the mksrcinfo (pkgbuild-introspection) command to check the .SRCINFO file or use the namcap command.
diff --git a/PKGBUILD b/PKGBUILD
index 0f06a4b..d7ad2af 100644
--- a/PKGBUILD
+++ b/PKGBUILD
@@ -19,7 +19,7 @@ makedepends=("broadcom-wl-dkms>=$pkgver"
"$_linuxprefix" "$_linuxprefix-headers")
groups=("$_linuxprefix-extramodules")
provides=("$_pkgname=$pkgver")
-backup=('etc/modprobe.d/$_linuxprefix-broadcom-wl.conf')
+backup=("etc/modprobe.d/$_linuxprefix-broadcom-wl.conf")
source=(broadcom-wl-dkms.conf)
sha256sums=('b97bc588420d1542f73279e71975ccb5d81d75e534e7b5717e01d6e6adf6a283')