The remote sshd process is not going to end the connection until all the subprocesses launched close their stdio streams.

Try redirecting stdout to /dev/null on the remote side:

qx/"D:\\plink -ssh $myhash{ROLE_ACCNT}\@$myhash{OSESSRVRS} -i $myhash{
+KEY} -t $myhash{OSES_HOME}\/startWebLogic.sh >\/dev\/null \&"/;
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