$re = qr  /(?:[\s\n]*|[\s]*%[^\n]*\n)*
     (?<first>               # start capture buffer 1?<first>
       \{                  #   match an opening paren
        (                   # capture buffer 2
        (?:                 #   match one of:
            (?>             #     don't backtrack over the inside of this group
                [^{}]+    #       one or more 
            )               #     end non backtracking group
        |                   #     ... or ...
            (?&first)            #     recurse to opening 1 and try it  again &first
        )*?                 #   0 or more times.
        )                   # end of buffer 2
        \}                  #   match a closing paren
     )                      # end capture buffer one
    /x;

$s =~ /\n[^%]*\\Befehl$re/g;