Hopefully in Go 1.14 there will be a new flag for go get that does exactly what you are asking. This is tracked in issue #30515 "cmd/go: offer a consistent global install command".
Prior to that, you have a few different options.
Go 1.12 and 1.13: change directory
If you are using Go 1.12 or later, the simplest solution is probably to move outside your current module to a directory without a go.mod prior to doing the go get, such as:
$ cd /tmp
$ go get github.com/foo/bar@v1.2.3
$ cd - # return to prior directory
Go 1.11, 1.12, 1.13+: gobin
gobin is a module-aware command to install or run binaries that provides additional flexibility, including the ability to install without altering your current module's go.mod. See the gobin README and FAQ for more details.
Go 1.11: temporary module
If you are using Go 1.11 with modules, the first step is probably to upgrade to Go 1.12 or 1.13 given there are many improvements in modules. If you are required to use Go 1.11 and want to use the @version syntax without updating your current module's go.mod, then one approach is to create a temporary module:
cd $(mktemp -d) && go mod init tempmod && go get github.com/foo/bar@v1.2.3
This is because in Go 1.11, you can't use the @version syntax unless you are in a module, which was relaxed in Go 1.12. This approach has been automated by a simple shell script by @rogpeppe.
Additional Details
In general, the go command in module-module always determines what module it is "in", which is based on the current working directory when you invoke the go command. (You could make an analogy to how make without any args will look for a makefile in the current working directory, or how historically go build without any args will build the current working directory, etc.).
With modules, go get looks for a go.mod file in the current working directory or any of its parents, and go get will use the constraints listed in any go.mod as part of solving for versions, as well as update the go.mod if needed based on doing the go get. That is why your go.mod file is updated if you run go get from within an existing module.
On the other hand, starting with Go 1.12, if you are in a directory that is not part of any module (that is, the directory does not have a go.mod, nor do any of its parents), then there is no go.mod to update, but the go command is still able to operate in module mode and use the @version syntax.
From the Go 1.12 release notes:
When GO111MODULE is set to on, the go command now supports module-aware operations outside of a module directory, provided that those operations do not need to resolve import paths relative to the current directory or explicitly edit the go.mod file. Commands such as go get, go list, and go mod download behave as if in a module with initially-empty requirements. In this mode, go env GOMOD reports the system's null device (/dev/null or NUL).
