The 5.6 version of perl should work, Solaris 7 (SunOS 5.7) has large file support and so does your perl. As far as how to proccess the file we really need more info on what you are trying to do, but if you want to keep memory usage kinda low you can always:

open (FILE, "a.large.file") or die "cant open a.large.file: $!\n";
while (<FILE>) { #read line by line so as not to slurp all of the file
+ into memory.
#do stuff to each line here
}
[download]

-Waswas
PS: upgrade to 5.6.1 it has been much better for me that 5.6.0 was =)