MouseClip is a resident program (TSR) that uses the mouse to copy and paste in DOS text modes. It can copy both blocks and streams, and has a configurable double-click mode. Text can also be pasted as a stream, and copied to and from the Windows clipboard. An additional feature is moving the screen cursor to the mouse cursor.
MouseClip v1.10 (26k, includes assembly source).
Similar programs you might want to look at are:
xPC-Mouse, very similar, but can write to a file. Also contains source.
MouseBuf, can take over programs that use the mouse.
Here's a link to a comparison chart.
25 September, 2002.