system("ssh -qv $ssh_user\@$bkp_destination 'sed -i \"s#.*$hostname#$skey#g\" /tmp/test.txt'");

Variables:
$hostname = "HOSTA";
$skey = "ssh-rsa AAAAB3NzaC1yc2 root@HOSTA";

SSH output:
...
debug1: Sending env LANG = en_GB.UTF-8
#ssh-rsa AAAAB3NzaC1yc2 root@HOSTA#g" /tmp/test.txt
...