GLM supports directly passing a GLM type to OpenGL using one of the OpenGL vector functions (with the v suffix). For example, to pass mat4 named proj to OpenGL, we can use the following code:

glm::mat4 proj = glm::perspective( viewAngle, aspect, nearDist, farDist ); 
glUniformMatrix4fv(location, 1, GL_FALSE, &proj[0][0]); 

Alternatively, rather than using the ampersand operator, we can use the glm::value_ptr function to get a pointer to the content of the GLM type:  

glUniformMatrix4fv(location, 1, GL_FALSE, glm::value_ptr(proj));

The latter version requires including the header file glm/gtc/type_ptr.hpp. The use of value_ptr is arguably cleaner, and works for any GLM type.