perl -pe 's/(\w)(\w+)(\w)/split"",$2;$,="";$,.=splice@_,rand@_,1while@_;$1.$,.$3/eg' file



##</code><code>##

 perl -pe 's/(?<=\w)\w+(?=\w)/split$,,$&;my$s;$s.=splice@_,rand@_,1while@_;$s/eg' file