Well, I don't have a sample of your input data to play with, but it don't see the point of the pack/unpack thing. It seems to me that the following would do what you want. If it doesn't can you tell me what you're acheiving with the pack/unpack thing?

sub split_up {
    my @d = @{ shift() };

    my @a = ($d[4] =~ /.{35}?/g);
    for (0..4) {
    $a[$_] = '' unless exists $a[$_]
    };

    my $cust  = join ',', map lc ('A', $d[0], 'WEB', '', @d[1..3],
                                  @a, @d[5..7], '', '', $d[8], '',
                                  $d[19], '', '', $d[18], '');

    my $child = join ',', map lc ($d[0], @d[9..17]);
    
    return ($cust, $child);
}
[download]

Update:
Well as dave pointed out (and I suggested may be the case), I totally missed the point of the pack/unpack. I looked into sprintf, but I'm not convinced it's any better than your original (I haven't tested this).

sub split_up {
    my @d = @{ shift() };

    my @a = ($d[4] =~ /.{35}?/g);
    for (0..4) {
        $a[$_] = '' unless exists $a[$_]
        };

    my $cu_fmt = '';
    for (1, 9, 3, 60, 10, 20, 40, 35, 35, 35, 35, 35, 10,
         3, 20, 20, 20, 50, 1, 1, 1, 1, 6, 6) {
        $cu_fmt .= "\%-${_}.${_}s,"
        }
    my $cust = sprintf $cu_fmt, ('A', $d[0], 'WEB', '',
                                @d[1..3], @a, @d[5..7],
                                '', '', $d[8], '', $d[19],
                                '', '', $d[18], '');

    my $child_fmt = '';
    for (9, 20, 20, 20, 20, 1, 1, 1, 6, 6, 6){
        $child_fmt .= "\%-${_}.${_}s,"
        }
    my $child = sprintf $child_fmt, ($d[0], @d[9..17]);

    return ($cust, $child);
}
[download]

Nuance

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