You could use your sigtrap fn to record the fact (in a global/pkg var) that the sig was received, then when convenient, use the perl built in 'kill' to send/fwd the prev received sig to whichever process.
It's not quite clear from your qn whether the child fork completes ever, or whether you mean when the shelled-out call completes...
Remember to clear the pkg var when you do deliver the sig.
Cheers
Chris