I've found the following answer regarding your question. According to the second paragraph, it's almost impossible to do what you want.
Answer by user grantm on perlmonks.org:
It may be possible - it depends
on how the PDF was generated. If the
option was used to convert text to
curves then you're probably out of
luck.
Also the PDF file format has no
concept of 'lines'. Characters in a
particular font family, weight, size
(etc) are placed at X/Y coordinates on
the page. They can be placed in any
order (ie: not just left-right, top to
bottom).
So if it were possible, one approach
would be to find the y coordinate of
every piece of text on the page;
reduce that to a unique set; sort them
and assign line numbers; go down and
add the line numbers as text elements
at the same y coordinate on the right
hand side of the page.
For the first part, you might find
that the CAM::PDF module has some
useful tools (eg: it can render just
the text elements of a PDF page).
Overlaying new text elements is the
easy part. I tend to use
PDF::Reuse but I'm sure that
PDF::API2 could be used too.
