This does the trick.

#! perl -slw
use strict;

my %desc;

m[^\s*(.*?)\s+([A-Z0-9]+)\s+(?:[0-9,]+\s*)?$]
    and $desc{ $2 } = $1
    while <DATA>;

my $code = 'GMF';
print "$code : $desc{ $code }";

__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
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