Strongs no. 622 perish is to "destroy fully". To perish or lose. Destroy, die,.
The wages of sin is
DEATH the gift of God eternal
LIFE through Jesus Christ our Lord.
Romans 6:23
Think "life". Then think the contrast between that and "death". Its the difference in having being and not having being.
The wicked will after going to the Gehenna fire pass into "outer darkness". Darkness is a scriptural metaphor for death.