What do you really want to generate? The Use of uninitialized value in sprintf is because you have two %ss in your format string, but only provide one string.

Do you mean something like this:

use warnings;
use strict;

my @rel = ("test1", "test2", "test3");

print join " &\n   ", @rel;
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which prints:

test1 &
   test2 &
   test3
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