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
   }
}