You can strip the IP address manually and only leave netmask with this one line.
:local a [:pick [/ip address get 0 address] ([:find [/ip address get 0 address] "/"] +1) [:len [/ip address get 0 address] ]];:put $a
Cleaner way ( in a script ):
:local address [/ip address get 0 address];
:local addressLen [:len $address];
:put [:pick $address ([:find $address "/"] +1) $addressLen];