:let @a=expand("%:p")

:              => Ex mode
let            => declare a variable
@a             => register 'a'
let @a         => Set register 'a' to whatever you like, e.g. let @a="Croatia"
expand({expr}) => Expand {expr} - it can be different form, e.g. "%:p" current file
Note :ls       => list the content of all registers, ofcuz @a too