I don't see why you need to change anything.
Regex in Perl 5.10 is a superset of 5.8.
I modified your code slightly but it will run under both versions.
I think should also run under Perl 5.6.
I see no need here for fancy new features past what has been done for more than a decade.
There are new things that can be done.
But you may not have understood what could have been done in your prior code.

use strict;
use Data::Dumper;

my %hash=();
my $str="Test Tester Testing [Feb 18: 28_10_10] Test";

my $reg='(\w+) (\w+) (\w+) \[([\w\W]+)\] (.*?)';

my ($fname, $mname, $lname, $date) =  $str =~ /$reg/is;
if (defined ($date))
{
   $hash{fname}=$fname;
   $hash{mname}=$mname;
   $hash{lname}=$lname;
   $hash{date} =$date;
}

print Dumper \%hash;
__END__
$VAR1 = {
          'date' => 'Feb 18: 28_10_10',
          'lname' => 'Testing',
          'mname' => 'Tester',
          'fname' => 'Test'
        };
[download]

Update:
If you must, although without spacing the regex out onto multiple lines, it is harder to understand...

use strict;
use Data::Dumper;

my %hash;
my $str="Test Tester Testing [Feb 18: 28_10_10] TestXXX";
my $reg='(?<fname>\w+) (?<mname>\w+) (?<lname>\w+) \[(?<date>[\w\W]+)\
+]';

if ($str =~ /$reg/i)
{
   %hash = %+;
}
print Dumper \%hash;
__END__
$VAR1 = {
          'lname' => 'Testing',
          'date' => 'Feb 18: 28_10_10',
          'fname' => 'Test',
          'mname' => 'Tester'
        };
[download]