From the description you gave this psuedo code may match the problem (yeah, I'm being hesitant)

if ( file1-field2 eq file2-field5 ) {
    give me file2-field_to_be_named || first_round_draft_pick
}
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OK, kidding about the draft pick.

If this is close to the truth, consider these fragments to work with:

my %file1;
my %file2;
while(<file1>) {
   my $field_from_1 = (split /<DELIMiTER>/)[1];
   $file1{$field_from_1}++;
}

while(<file2>) {
   my ($field_from_2, $record_we_want) = (split /<DELIMITER>/)[4,index
+_we_cant_guess_from_description];
   $file2{$field_from_2} = $record_we_want;
}

foreach my $keys ( %file2 ) {
   if ( exists $file1{$key} ) {
        # do whatever it is they want with $record_we_want
   }
}
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Those snips require a reasonably small subset of the memory needed to do what you described. Perhaps small enough to do the entire task in memory. What they don't do is:

• Preserve the order of the records, but we don't know if that's important from your description
• Preserve duplicate ID records, but we don't ...

If this is not helpful I'll join the first two responders in requesting you provide more information. A few lines of sanatized data file and a sample of the code you've developed so far will do wonders for our helpfulness.

Update: destressed some tortured English