Imperfect contrition means we are sorry for our sins, but our sorrow is based (at least in some small part) on fear of punishment.
Perfect contrition means we are sorry for our sins because we realize they offend God, not because we fear God’s judgement.
If a person suspects that he may be guilty of mortal sin, but cannot soon avail himself of Sacramental Confession, he may (if he is able) make an act of perfect contrition. BUT he must still avail himself of Confession at the nearest possible opportunity and confess this sin. If he dies before being able to do so, he is yet saved. If he neglects Confession, the act merits no Grace.
The real problem here is that it is theologically impossible to know if we are actually perfectly contrite. Such an act must be based entirely on love, without a hint of fear. Not even we ourselves can know if we have achieved this (it is known to God alone).
If bombs are falling, do what you can. It can’t hurt. But don’t look at it as any sort of assurance, because it is not. That’s why it is best to confess without delay (which gives 100% assurance).