The Collaborative International Dictionary of English v.0.48:

Imperishable \Im*per"ish*a*ble\, a. [Pref. im- not + perishable:
   cf. F. imp['e]rissable.]
   Not perishable; not subject to decay; indestructible;
   enduring permanently; as, an imperishable monument;
   imperishable renown. -- Im*per"ish*a*ble*ness, n. --
   Im*per"ish*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imperishable
    adj 1: not perishable [ant: perishable]
    2: unceasing; "an abiding belief"; "imperishable truths" [syn:
       abiding, enduring, imperishable]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "imperishable":
   amaranthine, deathless, embalmed, ever-fresh, everlasting,
   fadeless, frozen, immortal, impregnable, incorruptible, indelible,
   indestructible, ineffaceable, ineradicable, inexpugnable,
   inextinguishable, invincible, invulnerable, irrefragable,
   never-dying, nonperishable, quenchless, undampable, undestroyable,
   undying, unfading, unquenchable