A googling of "extract graph data from bitmap" yields a good few results. Sometimes you just need the right "search-engine-fu" to get what you're looking for. I didn't see any perl-specific bits, probably because I didn't search very hard. I just entered the first search phrase that popped into my head. But here are several candidates for you:

• There are third party solutions out there.
• The gnuplot docs contains a section for extracting data from graphs. The first link is to Engauge, an open source program. (The second link is broken...)
• Sourceforge has a project for this: Plot Digitizer that you may be able to use.
• If you're wanting to do it from scratch in perl, this thread shows an algorithm that might be a good starting point.

...roboticus

When your only tool is a hammer, all problems look like your thumb.