If you're going to roll your own shuffle, there's no reason you need to work on an array:

sub rand2 {
    my $foo = shift;
    for (my $i = length($foo); --$i; ) {
        my $j = int rand ($i+1);
        (substr($foo, $i, 1), substr($foo, $j, 1)) = (substr($foo, $j,
+ 1), substr($foo, $i, 1)) 
    }
    return $foo;
}
[download]

But then, there's no strong argument for shuffling in-place, either:

sub rand3 {
    my $foo = shift;
    my $bar = '';
    while (length $foo) {
      $bar .= substr($foo, rand(length $foo), 1, '');
    }
    return $bar;
}
[download]

Benchmark results:

         Rate rand1 rand2 rand3
rand1  2928/s    --  -27%  -78%
rand2  4008/s   37%    --  -70%
rand3 13282/s  354%  231%    --
[download]

Update: Benchmarked BrowserUK's shuffleword; it was about half as fast as rand3.