There is format and write, which are designed for that sort of output.

After Compline,
Zaxo

I am searching on GeneBank data, I got 480 Accession number, for each Accession number, I need two items: item1 and items, so I have the print format like:

Accession      item1       item2
12345678       abcd1234    als
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because I generate each accession number first, for each accession number, I use foreach loop to generate item1 and item2, maybe some of the accession number have a couple of groups item1 and item2, maybe some of them have none item1 and item2 at all. I tried to use:

printf ("%08d",$accession);#which is before foreach loop.
foreach (@accession){
....
$item1=....;
$item2=....;
printf("%16s%08s",$item1,$item2);
}
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It printed out so messy. Please help! Thanks!

Use a temporary variable. Set it to the value to print before entering the loop. After the first print in the loop, set it to an empty string. This assumes that the loop will produce at least one line of output.

my ($print_accession) = sprintf("%08s",$accession);
foreach (@accession) {
    ...
    $item1=...;
    $item2=...;
    printf("%8s%16s%8s\n",$print_accession,$item1,$item2);
    $print_accession = "";
}
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If you don't like your strings right aligned in their field, use '%-16s' - the minus indicates the field should be left aligned. See perldoc -f sprintf for details on the available formats.

90% of every Perl application is already written. ⇒

dragonchild

print <<'END';
1223455   abdkek    akdjeg
          ksitjls   jsoirl
          lsiet     oliut
2874824   lsiugl    potpsgj
          postis    potltids
1375080 
0875709   llskgjo   ooiwuto
9876653   lsgjgdlk  ldkgjldj
END
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-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

my %accession =
(
    '1223455' => [ { item1 => 'abdkek',   item2 => 'akdjeg' },
                   { item1 => 'ksitjls',  item2 => 'jsoirl' },
                   { item1 => 'lsiet',    item2 => 'oliut'  } ],
    '2874824' => [ { item1 => 'lsiugl',   item2 => 'potpsgj' },
                   { item1 => 'postis',   item2 => 'potltids' } ],
    '1375080' => [ ],
    '0875709' => [ { item1 => 'llskgjo',  item2 => 'ooiwuto' } ],
    '9876653' => [ { item1 => 'lsgjgdlk', item2 => 'ldkgjldj' } ]
);

my ($accession, $item1, $item2);

foreach $accession (sort keys %accession)
{
    #....

    if (not scalar @{$accession{$accession}})  # no items 
    {
        $item1 = $item2 = '';
        write;
        next;
    }

    foreach my $itemset (@{$accession{$accession}})
    {
        # Do whatever you want to set $item1, $item2

        $item1 = $itemset->{item1};
        $item2 = $itemset->{item2};
        write;
        $accession = '';  # blank for subsequent prints
    }
}

format STDOUT_TOP =
Accession      item1       item2
.

format STDOUT =
@<<<<<<<<<<<<< @<<<<<<<<<< @<<<<<<<<<
$accession,    $item1,     $item2
.
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