Something like this

 my @substrrefs = map {
    [  \substr( $text, $_ ) ,
      \substr( $text, 0, $_ )
    ]
} 0 .. (-1+length $text); 

my $bwt = join ...
     sort {
       $$a[0].$$a[1]
       cmp $$b[0].$$b[1]
    } @substrrefs;
[download]