According to the documentation you use eval()
to evaluate an expression.
register expr-register @r
--------
...
When using the '=' register you get the expression itself, not what it
evaluates to. Use eval() to evaluate it.
To pass the current line to eval()
you can use the following.
:s/.*/\=eval(submatch(0))
which replaces the expression with the result of running eval on it.
Take a look at :h sub-replace-expression