Here's one more (regex only) approach, which no one seems to have tried yet. I'm basing this on the assumption that the second column (which contains your intended hash key) is always separated from the first column by at least three spaces, whereas word separations within the first column are always single spaces:

#!/usr/bin/perl -w

use strict;

my %hash;
while (<DATA>) {
   s/^\s+//;  # remove leading whitespace
   if ( /(.*?)\s{3,}(\S+)/ ) {
      my ($val,$key) = ($1,$2);
      $hash{$key} = $val;
   }
}
print map { "$_ : $hash{$_}\n" } sort keys %hash;

__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__
B00004 : Manufacturing ("SIC")
B50001 : Total index
B5610C : Crude processing (capacity)
B562A3C : Primary & semifinished processing (capacity)
B5640C : Finished processing (capacity)
G321 : Wood product
G327 : Nonmetallic mineral product
G331 : Primary metal
G3311A2 : Iron and steel products
G332 : Fabricated metal product
G333 : Machinery
GMF : Manufacturing (NAICS)
GMFD : Durable manufacturing (NAICS)
[download]

I have to confess, I've been pretty slow to get comfortable with unpack(), myself. It is certainly one of the more difficult functions to grasp (and its description in perfunc is still a bit hard to follow).