Sorry kiat, seems I completely misunderstood the task
Here's another go, though tachyon's solution is excellent:

#!/usr/bin/perl -w
use strict;

my $bold = "[b]bold text[/b]";
my $red = "[color=Red]Red text text[/color]";
my $red_bold = "[color=Red][b]Red bold text[/b][/color]";
my $empty = "[color=Red][b][/b][/color]";

&check_tags($bold);
&check_tags($red);
&check_tags($red_bold);
&check_tags($empty);

sub check_tags {
  my $tag = shift;
  print $tag, $/ if $tag =~ /(?:\[[^\]]+\])+.+?(?<!\])(?:\[\/).+/;
}

__END__
__OUTPUT__
[b]bold text[/b]
[color=Red]Red text text[/color]
[color=Red][b]Red bold text[/b][/color]
[download]