Using vi(m) you can insert the output of an external command easily using this vi-command:

:.!myscript.pl
[download]

and of course you can use an external script to modify specified lines in your actual buffer:

:1,$!sort
[download]

This Wikipedia-article shows a lot of the powerful features you can use within vi(m).
You can use :help command to get help directly in vi(m).

UPDATE: Changed link to non-german Wikipedia.