I think this is a perfectly valid place to undefine a variable. You could also use my to redeclare the variable, but there is no need to declare the variable both inside the loop and outside:

my @final;
my @array = qw(foo bar baz);

for (0..$#array) {
   my $temp;
   for my $j (0..$#array) {
      $temp .= $array[$j];
   }
   push @final, $temp;
}
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Remember, undef is not bad, you just have to know when it is a good choice:

my $temp;
for (0..$#array) {
   for my $j (0..$#array) {
      $temp .= $array[$j];
   }
   push @final, $temp;
   undef $temp;
}
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Here, it is perfectly fine because we are undefining a scalar. But, what happens if we push another array instead?

for my $i (0..$#array) {
   my @temp = @array;
   push @final, \@temp;
   undef @temp;
}
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This is a bad choice. Now @final contains three anonymous, empty array references. What happened? We copied the contents of @array, but after we pushed our temp array, we undefined it - hence, we 'erase' the data we just pushed. But, had we pushed a new anonymous array reference that contained our temp array, then it works:

for my $i (0..$#array) {
   my @temp = @array;
   push @final, [@temp];
   undef @temp;
}
[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)