Are you sure you're calling the right Perl version? Grepping through the 5.8.8 Perl sources doesn't locate any such message, so it's unlikely it's issued by this version of Perl.  OTOH, Solaris' system Perl is typically ancient (e.g. 5.005_03 on Solaris 8), so my guess would be that you're inadvertently calling this system Perl...

5.005_03

$ /usr/bin/perl       -e 'open my $fh, "uname -rs |"; print <$fh>;'
Can't use an undefined value as filehandle reference at -e line 1.
[download]

5.8.8

$ /usr/local/bin/perl -e 'open my $fh, "uname -rs |"; print <$fh>;'
SunOS 5.8
[download]