Here is an example using -i. Notice that a new file is created with the same name. This isn't quite the same as editing the same file. You could have problems if there are multiple hard links to the file you are editing.

[chad@desktop ~]$ ls -li asdf
2507085 -rw------- 1 chad chad 10 2010-09-17 10:44 asdf

[chad@desktop ~]$ cat asdf
1
2
3
4
5

[chad@desktop ~]$ perl -p -i.bak -e 's/^2$/two/' asdf

[chad@desktop ~]$ cat asdf
1
two
3
4
5

[chad@desktop ~]$ cat asdf.bak 
1
2
3
4
5

[chad@desktop ~]$ ls -li asdf asdf.bak 
2507203 -rw------- 1 chad chad 12 2010-09-17 10:46 asdf
2507085 -rw------- 1 chad chad 10 2010-09-17 10:44 asdf.bak

[chad@desktop ~]$ perl -p -i -e 's/^2$/two/' asdf

[chad@desktop ~]$ ls -li asdf asdf.bak 
2507214 -rw------- 1 chad chad 12 2010-09-17 10:49 asdf
2507085 -rw------- 1 chad chad 10 2010-09-17 10:44 asdf.bak

[chad@desktop ~]$
[download]

Update: the sed command also has a -i flag.