Built-in functions are not available in certain contexts (impl. details).

Example:

@bar: pi();
@import "foo-@{bar}.less";

Result:

SyntaxError: Cannot read property 'get' of undefined in *.less on line 2, column 1

The error simply means that the pi function is undefined at that point.
This happens because each context/frame has its own copy of the functions set, but the set is copied there too late to be available for a (preeval only?) visitor (e.g. like the import visitor above). This obviously prevents any (preeval?) visitor to be able to evaluate any code involving a function call.
Not sure if it still applies to the 3.x branch.

Start by tracing how each context/frame copies the functions set and how the import or other preeval visitor evaluates the example using pi(). Reproduce the reported SyntaxError, then verify that built-in functions are available when function calls are evaluated in those contexts.