Strange. Nobody used bitwise or. The following is 41 chars:

sub R1{
    $_=sub{join"\0",split//,pop};"\0".&$_|&$_
}
[download]

This is 37 chars but would clobber a subroutine called "_":

sub R2{
    sub _{join"\0",split//,pop}"\0".&_|&_
}
[download]

This is 33 chars but doesn't preserve case:

sub R3{
    sub _{join$",split//,pop}$".&_|&_
}
[download]

John.
--