code segfaults sometimes......Why?

I can't say without seeing some code, and you may be better off asking on the Perl/Gtk2 maillist, but they will want a simple example also.

Try to use the latest version of Cairo, and look at the examples. For example, regarding your yellow problem, the module's /examples/png subdir shows a stars.pl example, which shows setting color by

 $cr->set_source_rgb (0, 0, 0);
[download]

to set the color,

and the perldoc shows 2 ways, 1 with an alpha layer

$cr->set_source_rgb ($red, $green, $blue)
           $red: double
           $green: double
           $blue: double
[download]

for rgb32 you get an alpha layer

 $cr->set_source_rgba ($red, $green, $blue, $alpha)
           $red: double
           $green: double
           $blue: double
           $alpha: double
[download]

Now your setting $data with

$data .= pack 'CCCC',255,$value,$value,$value; where $value is int from 0 to 255

dosn't seem to to be doing what is needed to set the stroke color....you almost appear to have the alpha setting (255) at the wrong end of the list, and additionally I'm not sure pack 'CCCC' is the right pack option.