So if the data format really _is_ fixed-width columns then something like dragonchild's code would work, using

  my @column_widths = (57, 17, '*');
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for the widths (check against the real column widths).   Although to remove the leading _and_ trailing spaces from each piece I'd do something like:

  my ($desc, $code, $other_thingy) = unpack $unpack_spec, $_;
  foreach my $piece ($desc, $code, $other_thingy) {
    $piece =~ s/^\s+//;
    $piece =~ s/\s+$//;
  }
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(I think that's right, hmmm, testing with dragonchild's modified code ....)

# Change these to the actual column widths. Use a star at the end to g
+et the rest.
my @column_widths = ( 57, 17, '*');
my $unpack_spec = join ' ', map { "A$_" } @column_widths;

my %codes;
while (<DATA>)
{
    chomp;
    my ($desc, $code, $other_thingy) = unpack $unpack_spec, $_;
    foreach my $piece ($desc, $code, $other_thingy) {
      $piece =~ s/^\s+//;
      $piece =~ s/\s+$//;
    }

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

my  $choice = 'GMF';
print "$choice: $codes{$choice}{Description}\n";
    $choice = 'G3311A2';
print "$choice: $codes{$choice}{Description}\n";

__DATA__
Total index                                              B50001
 Crude processing (capacity)                             B5610C
 Primary & semifinished processing (capacity)            B562A3C
 Finished processing (capacity)                          B5640C
 Manufacturing ("SIC")                                   B00004
  Manufacturing (NAICS)                                  GMF
   Durable manufacturing (NAICS)                         GMFD
    Wood product                                         G321         
+    321
    Nonmetallic mineral product                          G327         
+    327
    Primary metal                                        G331         
+    331
     Iron and steel products                             G3311A2      
+    3311,2
    Fabricated metal product                             G332         
+    332
    Machinery                                            G333         
+    333

_ _ OUTPUT _ _ 

GMF: Manufacturing (NAICS)
G3311A2: Iron and steel products
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