If you only need 34 of 170 fields, you can use x[nnn] to skip the bytes between the fields you want. That should reduce the unweildy length of your template considerably.

And if the template is still uncomfortably long:

my $fmt = join '', qw[
    x[10] a5
    x[22] a10
    x[4]  a3 a2
    ...
];
[download]

You can even turn that into an opportunity to document:

my $fmt = join '', grep !m[^#], qw[
    x[10] a5     #code
    x[22] a10    #thingummy
    x[4]  a3 a2  #doodah&whatsit
    ...
];
[download]

As for the naming of the fields, rather than using strings of "FIELDnn" in a hash, why not use an array?

Eg.

my $fmt = "x[10]a5x[22]a10x[4]a3a2...";
my %hash;
while( <$fh> ) {
    my( $code, @fields ) = unpack $fmt, $line;
    $hash->{$code} = \@fields;
}
...
for my $code ( keys %hash ) {
    print $hash->{ $code }[ $_ ] for 0 .. 33;
}
[download]