Off hand code:

    while ( <FILE1> ) {
        $key = extract_key_from_line( $_);
        $file1_keys{$key} ++;
    }
    while ( <FILE2> ) {
         $check_me = extract_key_from_line( $_);
          do_something( $check_me) if exists $file1_keys($check_me);
    }
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