emcas replace-rectangle

Command    Key    Purpose
replace-rectangle    【Ctrl+x r t】    Replace text in a selected column.

try change this

 3c21 646f 6374
 6874 6d6c 3e3c
 2063 6861 7273
 202f 3e0a 3c6d
 6965 7770 6f72
 2277 6964 7468

to this

 3c21 cat 6374
 6874 cat 3e3c
 2063 cat 7273
 202f cat 3c6d
 6965 cat 6f72
 2277 cat 7468

    Put your cursor before “646f”.
    Call set-mark-command 【Ctrl+Space】.
    Move cursor to end of “6964”.
    Call replace-rectangle 【Ctrl+x r t】.