It's not clear from your code what format you're using for $target (the get() functions accepts a bunch of different input formats) - it might be easiest to use an SNMP::Varbind object as the input for get(). An SNMP::Varbind object is a blessed reference to an array with 5 elements:
- object name (e.g. "sysdesc" or ".1.3.6.1.2.1.1.1"
- dotted-decimal instance ID (or "0" for scalar objects)
- value
- value type (e.g. "IPADDR", "COUNTER", etc.)
- timestamp

You can set the name of the object, and get() will populate the rest of the info, which you can then test to figure out what format the return value is in. Assuming your input target is a string like "sysDescr" or ".1.3.6.1.2.1.1.1", you should be able to do something like:

my $var = new SNMP::Varbind( [ $target ] );
my $value = $session->get($var)
      || warn "SNMP ERROR: $session->{ErrorStr}";
print "name: ", $var->name, "\n";
print "type: ", $var->type, "\n";
if ( $var->type eq "IPADDR" ) {
   ... unpack $var->val here ...
}
elsif ( $var->type eq "COUNTER" ) {
   ... unpack $var->val here ...
}
elsif ...
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The SNMP man page will list all the valid variable types that you might need to deal with.