When in Solarisland, ask Solaris (pflags).

This one is ultra-fragile, but decide yourself if you can use or expand it!

use strict;
use warnings;

#-- returns 32 or 64, or 0 in case of error 
#   optional argument: PID of another process; defaults to current PID
#   see also: http://docs.oracle.com/cd/E26502_01/html/E29030/pflags-1
+.html      (pflags)
#             http://docs.oracle.com/cd/E26502_01/html/E28556/proc-pro
+vider.html (psinfo_t section)
#   "...The pr_dmodel field is set to either PR_MODEL_ILP32, denoting 
+a 32bit process,
#       or PR_MODEL_LP64, denoting a 64bit process."
#
#   WARNING! Tested under Solaris 10/11 only!
#
sub solaris_bits {
        my $pid    = shift || $$;
        my $pflags = qx{LANG=C /usr/bin/pflags $pid 2>&1}; 

        #-- "data model" should be '_ILP32' or '_LP64'
        return  $pflags =~ /data model\s*=\s*\D*(\d+)/ms  ?  $1 : 0;
}


print "Bits (init): ", solaris_bits(1) || 'n/a' , "\n"; # PID==1 (need
+s privileges!)
print "Bits (self): ", solaris_bits()  || 'n/a' , "\n"; # PID==$$
[download]

However, when checking bit-width for the current perl process ($pid == $$), I would rather use BrowserUks pack() approach.
There is also Solaris::Procfs but the bug-reports indicate, that it doesn't compile in a 64-bit environment. Nice opportunity to make this module work again (wink).