There are all kinds of interesting games you can play with pack, and it should be easy to encapsulate your solution in a function (with lotsa validation) so that maintainers only see data. See below.

Note that the  @ pack template specifier fills with nulls, so if you want space-filling, you need the
    $packed =~ tr{\000}{ };
statement. Note also that  @ is absolute, so maybe play around and see, e.g., what happens if the  date offset is set to 1 or 2.

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"use constant ORDER => qw(date  client_id  info);
 ;;
 my %o_l = (
   date      => { qw(off  0  len 6  field 040814)             },
   client_id => { qw(off  6  len 8  field WayTooMuchClientID) },
   info      => { qw(off 20  len 7  field G1APD)              },
   );
 ;;
 my $fields =
   join '  ',
   map  qq{\@$o_l{$_}{off} A$o_l{$_}{len}},
   map  { exists $o_l{$_} or die qq{bad '$_'};  $_; }
   ORDER
   ;
 ;;
 my $packed = pack $fields, map $o_l{$_}{field}, ORDER;
 $packed =~ tr{\000}{ };
 print 'packed len ', length $packed, qq{ '$packed'};
 ;;
 my $total = 30;
 die 'truncation!' if length($packed) > $total;
 my $line = pack qq{A$total}, $packed;
 print 'total  len ', length($line), qq{ '$line'};
 ;;
 dd \$line;
"
packed len 27 '040814WayTooMu      G1APD  '
total  len 30 '040814WayTooMu      G1APD     '
\"040814WayTooMu      G1APD     "
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