Perseverance is tied to Election in the mind of the Calvinist, and is essentially about Eternal Life.
Here is an excerpt from Millard Erickson’s Christian Theology (Vol. III, 983-987). Erickson clearly understands the essence of the matter:
The Calvinist position is both clear and forthright on this matter: “They whom God hath accepted in His Beloved, effectually called and sanctified by His Spirit, can neither totally nor finally fall away from the state of grace; but shall certainly persevere therein to the and, and be eternally saved.” [Westminster Confession] This point is consistent with the remainder of the Calvinist theological system. Since God has elected certain individuals out of the mass of fallen humanity to receive eternal life, and those so chosen will necessarily come to receive eternal life, it follows that there must be a permanence to their salvation. If the elect could at some point lose their salvation, God’s election of them to eternal life would not be truly effectual.