Don't bother.
The underlying FLTK 2.0 library is dead. When I inherited the project, it was based on the 2.0 branch which was abandoned upstream in 2011. They refuse to even distribute snapshots of the source which is why Alien::FLTK doesn't build and my work (which was properly licensed) is only found on backpan now. I planned to move to 1.3.x (which was created after 2.0) with
https://metacpan.org/release/Fl but it was also dropped upstream, then up to 3.0 in 2011-2012 which was also later dropped upstream, then 1.4 in 2016 which is still incomplete.
That's confusing, I know... Here's a commit timeline for the different branches:
http://www.fltk.org/svn_monthly_commit_activity.svg
Every branch created and later abandoned is incompatible with the other branches because they redesign the entire API every single time.
See also:
http://www.fltk.org/articles.php?L825
http://www.fltk.org/software.php