Copy file to protected directory using AppleScript
Got it...
tell application &quot;Finder&quot;
    set posixSource to (POSIX file &quot;/Users/darwin/Desktop/test.txt&quot; as alias)
    set posixDest to (POSIX file &quot;/Applications/My Special App.app/Contents/&quot; as alias)
    duplicate file posixSource to folder posixDest with replacing
end tell

... and the one-liner:
tell application &quot;Finder&quot; to duplicate file (POSIX file &quot;/Users/darwin/Desktop/test.txt&quot; as alias) to folder (POSIX file &quot;/Applications/My Special App.app/Contents/&quot; as alias) with replacing

Some important distinctions from failed attempts:

You can't alias a file which doesn't yet exist.  For destination, use the destination's parent folder instead.
You can't copy into the root of an Application bundle (e.g. My Special App.app).  You must copy into the My Special App.app/Contents instead.
Pay special attention to the keywords file and folder as they're provided to the duplicate command.
When a problem occurs, it can deadlock Script Editor.  Wrap your duplicate call in ignoring application responses [...] end ignoring to speed up testing which results in timeouts.
When converting to a one-liner you'll need to use the keyword to after the application name.