open IN ,"a.txt" or die $!; ##读入文件
open OUT ,">b.txt" or die $!; ##输出文件

while (<IN>) {

print OUT $_;
if ($_ =~ /add/){print OUT "xxx\n"
}
}

[root@june2 perl]# cat a.txt

add

1

2

3

4

add

5

6

add

8

9

aaaa

bbbbb

[root@june2 perl]# perl a1.pl

[root@june2 perl]# cat b.txt

add

xxx

1

2

3

4

add

xxx

5

6

add

xxx

8

9

aaaa

bbbbb