This is how I implemented BW for Rosalind.info:

my @suff = 0 .. length $string;
pop @suff;
@suff = sort { substr($string, $a) cmp substr $string, $b } @suff;

for my $idx (@suff) {
    print substr $string, $idx - 1, 1;
}
print "\n";
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Update: Now I noticed they mentioned O(|Text|) solution wasn't needed in the Note.