#pseudo code
open lookup file
while(<lookup>) {
chomp;
my ($find,$replace)=split /=/;
$hash{$find}=$replace;
}
close lookup

open file to work on
while(file) {

my $field_to_lookup_on=split/unpack whatever

if(exists $hash{$field_to_lookup}) {
$new_field=$hash{$field_to_lookup};
}
else {
$new_field=$field_to_lookup_on;
}

put field back in record
print record.