$ perl -Mstrict -Mwarnings -E '
    my $x = "[8/29/2013 7:16:45 AM] User abc def [DEFAULT]";
    $x =~ s{\[([^\]]*)\]}{"[" . do { (my $y = $1) =~ y/ /\t/; $y } . "]"}eg;
    say $x;
' | cat -vet
[8/29/2013^I7:16:45^IAM] User abc def [DEFAULT]$