As this is fixed length, use unpack. Now, you're going to have to also use some logic if you care about the indenting stuff to make sure that you don't keep that whitespace at the beginning of your description. If you don't, it's easy enough to strip off the indenting whitespace. To get what you exactly wanted, do something like:

# Change these to the actual column widths. Use a star at the end to g
+et the rest.
my @column_widths = (###, ###, '*');
my $unpack_spec = join ' ', map { "A$_" } @column_widths;

my %codes;
while (<IN_FILE>)
{
    chomp;
    my ($desc, $code, $other_thingy) = unpack $unpack_spec, $_;

    # If you want to remove the pre-pended whitespace on the descripti
+on ...
    $desc =~ s/^\s+//;

    $codes{$code} = {
        Description => $desc,
        Other_Thing => $other_thingy,
    };
}

my $choice = 'GMT';
print "$choice: $codes{$choice}{Description}\n";
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------
We are the carpenters and bricklayers of the Information Age.

The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6

Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.