#!/usr/bin/env perl -l

use strict;
use warnings;

my ($mdy, $hms, $CLIENT_ID) = qw{MMDDYY HHMMSS 123};
my @fields = (
    [135, ''], [11, "DATE=$mdy"], [6, $mdy], [6, $hms],
    [3, ''], [17, ''], [8, 'G1ADP'], [3, $CLIENT_ID],
);
my $line = pack 'A510';
my $offset = 0;
add_field(\$line, \$offset, @$_) for @fields;

print 'Length of line = ', length $line;
print "Line = |$line|";

sub add_field {
    my ($line_ref, $offset_ref, $length, $data) = @_;

    substr($$line_ref, $$offset_ref, $length)
        = $data . ' ' x ($length - length $data);
    $$offset_ref += $length;
}