This is mentioned in the README.solaris coming perl 5.8.1:

=head1 RUNTIME ISSUES FOR PERL ON SOLARIS.

=head2 Limits on Numbers of Open Files on Solaris.

The stdio(3C) manpage notes that for LP32 applications, only 255  
files may be opened using fopen(), and only file descriptors 0
through 255 can be used in a stream.  Since perl calls open() and
then fdopen(3C) with the resulting file descriptor, perl is limited
to 255 simultaneous open files, even if sysopen() is used.  If this
proves to be an insurmountable problem, you can compile perl as a
LP64 application, see L<Building an LP64 perl> for details.  Note
also that the default resource limit for open file descriptors on
Solaris is 255, so you will have to modify your ulimit or rctl
(Solaris 9 onwards) appropriately.
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Abigail