When you use Control+V in a single-line editbox, all of the text you are pasting is parsed as a command if it begins with / or as normal text if not. So "/aliasname {" is parsed as a command.

When you use Control+V in a multi-line editbox, all of the text is entered into the editbox and you can then either press enter or Control+Enter.

Control should only be a modifier to force plain text when used with Control+Enter, as documented in the help file.