Hmm... I don't think it has to be clumsy. Here's how I did it:

use strict;

my $string = 'My name is Bob.';
my $percent = 0.5;
my @obs = ('a'..'z');  # This array better be larger that length($stri
+ng) * $percent

my @stringArray = split(//, $string);        # break the string up
my @positions = (0..(length($string) - 1));  # the locations that will
+ be swapped
my $totalSwaps = int(length($string) * $percent);

srand(1);
_shuffle(\@positions);
_shuffle(\@obs);

for (my $i = 0; $i < $totalSwaps; ++$i) {
    $stringArray[$positions[$i]] = $obs[$positions[$i]];
}

$string = join('', @stringArray);
print $string . "\n";

sub _shuffle($)
{
    my $aref = shift;
    my $pos;
    for(my $i = 0; $i < scalar(@$aref); ++$i) {
        $pos = int(rand(scalar(@$aref)));
        ($aref->[$i], $aref->[$pos]) = ($aref->[$pos], $aref->[$i]);
    }
}
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-Ton
-----
Be bloody, bold, and resolute; laugh to scorn
The power of man...