Perhaps there is a confusion. Your subroutine db_retrieve_records returns a reference to the @loop array, not the @loop array itself. If that is the return value you want for db_retrieve_records, then the mail routine needs correcting.

First, save the reference to @loop:

my $loop = db_retrieve_records();
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Then access loop information with the -> dereferencing op:

for my $i (0..@$loop-1) {
    foreach (qw( action type domain priority target )) {
        $body .= $loop->[$i]{$_};
    }
    $body .= "\n";
}
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Note also that I used @$loop-1 for the last element of the outer loop.

UPDATE: Mr. Freidman's is a much simpler fix. Consider mine as an alternative way to do it :)

-Mark