I can confirm the problem.

Actually, I've installed the module and played around with it a bit, but unfortunately, I have no good news... i.e. although you can fix the immediate issue with libperl.so not being found by adding /usr/perl5/5.8.4/lib/sun4-solaris-64int/CORE to LD_LIBRARY_PATH, it doesn't really help, because then you'll (most likely) run into the next problem with a bunch of symbols not being found:

$ echo $LD_LIBRARY_PATH
/home/almut/tmp/STAF/lib/:/usr/perl5/5.8.4/lib/sun4-solaris-64int/CORE

$ /usr/bin/perl -MPLSTAF -e1
ld.so.1: perl: fatal: relocation error: file /home/almut/tmp/STAF/lib/
+/libPLSTAF.so: symbol Perl_Tstack_sp_ptr: referenced symbol not found

/home/almut/tmp/STAF/lib $ ldd -r libPLSTAF.so
        libpthread.so.1 =>       /lib/libpthread.so.1
        libsocket.so.1 =>        /lib/libsocket.so.1
        libresolv.so.2 =>        /lib/libresolv.so.2
        libSTAF.so =>    /home/almut/tmp/STAF/lib//libSTAF.so
        libperl.so =>    /usr/perl5/5.8.4/lib/sun4-solaris-64int/CORE/
+libperl.so
        libstdc++.so.2.10.0 =>   /home/almut/tmp/STAF/lib//libstdc++.s
+o.2.10.0
        libm.so.1 =>     /lib/libm.so.1
        libnsl.so.1 =>   /lib/libnsl.so.1
        libc.so.1 =>     /lib/libc.so.1
        libdl.so.1 =>    /lib/libdl.so.1
        libm.so.2 =>     /lib/libm.so.2
        libmp.so.2 =>    /lib/libmp.so.2
        libmd5.so.1 =>   /lib/libmd5.so.1
        libscf.so.1 =>   /lib/libscf.so.1
        libdoor.so.1 =>  /lib/libdoor.so.1
        libuutil.so.1 =>         /lib/libuutil.so.1
        symbol not found: boot_DynaLoader               (./libPLSTAF.s
+o)
        symbol not found: Perl_sys_init3                (./libPLSTAF.s
+o)
        symbol not found: Perl_Tstack_sp_ptr            (./libPLSTAF.s
+o)
        symbol not found: Perl_Tmarkstack_ptr_ptr               (./lib
+PLSTAF.so)
        symbol not found: Perl_Tstack_base_ptr          (./libPLSTAF.s
+o)
        symbol not found: Perl_croak_nocontext          (./libPLSTAF.s
+o)
        symbol not found: Perl_Imodglobal_ptr           (./libPLSTAF.s
+o)
        symbol not found: Perl_hv_common_key_len                (./lib
+PLSTAF.so)
        symbol not found: Perl_sv_2uv_flags             (./libPLSTAF.s
+o)
        symbol not found: Perl_Isv_yes_ptr              (./libPLSTAF.s
+o)
        symbol not found: Perl_Ttmps_floor_ptr          (./libPLSTAF.s
+o)
        symbol not found: Perl_Ttmps_ix_ptr             (./libPLSTAF.s
+o)
        symbol not found: Perl_Tmarkstack_max_ptr               (./lib
+PLSTAF.so)
        symbol not found: Perl_Ierrgv_ptr               (./libPLSTAF.s
+o)
        symbol not found: Perl_TXpv_ptr         (./libPLSTAF.so)
        symbol not found: Perl_newSVpvf_nocontext               (./lib
+PLSTAF.so)
        symbol not found: Perl_load_module_nocontext            (./lib
+PLSTAF.so)
        symbol not found: Perl_Gthr_key_ptr             (./libPLSTAF.s
+o)
        symbol not found: Perl_Iperl_destruct_level_ptr         (./lib
+PLSTAF.so)
        symbol not found: Perl_Iexit_flags_ptr          (./libPLSTAF.s
+o)
        symbol not found: Perl_Tstack_max_ptr           (./libPLSTAF.s
+o)
        symbol not found: Perl_sv_2iv_flags             (./libPLSTAF.s
+o)
        symbol not found: Perl_Top_ptr          (./libPLSTAF.so)
        symbol not found: Perl_Tcurpad_ptr              (./libPLSTAF.s
+o)
        symbol not found: Perl_Isv_undef_ptr            (./libPLSTAF.s
+o)
        /platform/SUNW,Sun-Blade-2500/lib/libc_psr.so.1
        /platform/SUNW,Sun-Blade-2500/lib/libmd5_psr.so.1

# confirmed by checking with nm, e.g.

$ nm -D ./libPLSTAF.so | grep Perl_Tstack_sp_ptr
[41]    |         0|       0|FUNC |GLOB |0    |UNDEF  |Perl_Tstack_sp_
+ptr

$ nm -D /usr/perl5/5.8.4/lib/sun4-solaris-64int/CORE/libperl.so | grep
+ Perl_T
[download]

AFAICT, most of these symbol errors (if not all - I'm not entirely sure) are related to the Solaris' system perl not being built with threading support, while the perl that the module had been built against was...

So, your options now seem to be:

• build an appropriate perl with threading support
• download the STAF sources and try to build the module yourself (in the hopes that threading support isn't really required...)