perl -pe

's|(\w)(\w+)(?=\w)|$1.join"",sort{rand 2}split//,$2|ge'

 12345678901234567890123456789012345678901234567890123
          10        20        30        40        50
Edit:
perl -pe
's|\B(\w+)\B|join"",sort{rand 2}split//,$1|ge'
[download]

The golf has more perceptually random results than the other variants posted here which are shorted than it. It doesn't have an even distibution of results tho; words longer than 5 characters have certain permiatations which are more likely to come up. There's an example in the <readmore>.

xxx