How about this? This parallels the stata code closely.
variables_to_cap = [:price, :mpg, :headroom, :trunk, :weight]
for variable in variables_to_cap
newvar = Symbol(string(variable,"_std"))
q = quantile(df[variable],.99)
df[newvar] = [!isna(x) && x > q ? q : x for x in df[variable]]
end