JOML : Perspective Matrix doesnot retain shape

Question

Im trying to make my own 3D engine using LWJGL.
This is the Line of Code which creates the projection matrix for the uniform to pass.
pMatrix = new Matrix4f().ortho(-2f, 2f, -1.125f, 1.125f, 0.1f, 1000f);
Perfectly working Orthographic Projection
Notice how the rendered mesh is square, correct according to the vertices
Only changing this one line to
pMatrix = new Matrix4f().perspective((float) Math.toRadians(60.0f), 640/360,0.1f, 1000f);
Breaks It
Not Properly working Perspective projection
Notice how in the second image , the mesh is not square
Is there something that im doing wrong. If yes , please help me fix it. If not then why is this happening.