Because your data files are very simple, only one hash table is required to hold the industry lookup. I have written a quick and simple code to do what you described. Keep the script simple and straight forward.

use strict;
use IO::File;

my %industries;

# Load the industry definition file
my $def = new IO::File "def.txt", "r" or die "Can not open file!";
while (<$def>) {
    if (m/'(.*?)','(.*?)'/) {
        $industries{$1} = $2;
    }
}
undef $def; # close the file

# Load the data file
my $dat = new IO::File "dat.txt", "r" or die "Can not open file!";
while (<$dat>) {
    s/('.*?'),('.*?')/$2,$1/g; # swap the items
    my $id = substr($_, 1, 2);
    print "'$id','$industries{$id}'|$_";
}
undef $dat;
[download]

The input files are -

-- def.txt --
'BB','Drinks Industry'
'BL','Construction Industry'

-- dat.txt --
'Coffee Suppliers','BB100'|'Tea Suppliers','BB106' 
'Plasterers','BL100'|'Fencing Companies','BL102'
[download]

And the output is -

'BB','Drinks Industry'|'BB100','Coffee Suppliers'|'BB106','Tea Supplie
+rs'
'BL','Construction Industry'|'BL100','Plasterers'|'BL102','Fencing Com
+panies'
[download]

Cheers. Roger