I took jlongino's suggestion and reworked it a little bit. I moved the padding into a subroutine and made it so it only creates enough extra whitespace to satisfy the format condition.

$c = "test test test";

$c = correct_length($c);

$fmt = "A4 x1 A4 x1 A4 x1 A4";
@c = unpack($fmt, "$c");
print join ("*",@c), "\n";

$d = "test test test ";
$d = correct_length($d);
$fmt = "A4 x1 A4 x1 A4 x1 A4";
@d = unpack($fmt, "$d");
print join ("*",@d), "\n";

sub correct_length {
    my ($string) = shift;
    $string .= ' ' unless length($string) >= 15;
    return($string);
}
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This is assuming all your widths are the same, otherwise you would want to either pass the required width as an argument to the sub (not a good idea if you use the size in multiple places) or create a hash with values and there corresponding lengths and pass the item_name. I think this would give it a little more durability. Some thing like this:

my %formats = (
   "item_name"  => 15, # where 15 is the length
   "item_name2" => 20,
   # etc.
);

# code removed for demonstration

$c = correct_length($c,'item_name');

sub correct_length {
    my ($string,$item) = @_;
    $string .= ' ' unless length($string) >= $formats{$item};
    return($string);
}
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Then the subroutine above could be passed the item_name and the string so the only place you would have to modify length would be in the formats hash, but I don't know if your data is that complex/varied.