open '+<', '<file.txt';

##</code><code>##

1
2
3
4
5

##</code><code>##

1
two
hi hello
3
4
5

##</code><code>##

open FH, '+<file.txt';

while (<FH>)  {
        print $_;
        if ( /^2/ )  {
        print FH "two\nhi hello\n";
        }
}

##</code><code>##

1
2
two
hi hello