use strict;
use warnings;

my $input = '[8/29/2013 7:16:45 AM] User abc def [DEFAULT] [8/29/2013 7:16:45 AM] User xyz abc [DEFAULT]';
my $output = join '', map { s/\s/\t/g if /\[/; $_ } split /(\[.*?\])/, $input;
print "$output\n";