It's easier to get the opening and closing tags with the enclosed text in one go instead:

#!/usr/bin/perl

use strict;
use warnings;

$/ = undef;
my $text = <DATA>;
$text =~ s!\[color=([^\]]+)\](.*?)\[/color\]!<font color="$1">$2</font
+>!gi;
$text =~ s!\[image=([^\]]+)\]!<img src="$1">!gi;
$text =~ s!\[link=([^\]]+)\](.*?)\[/link\]!<a href="$1">$2</a>!gi;
print $text;

__DATA__
[color=#0000ff]Blue text[/color] with an image: [image=var.jpg]
And a link to Google: [link=http://www.google.com]Google[/link].
[download]

This gives the following output (which I think is what you want):

<font color="#0000ff">Blue text</font> with an image: <img src="var.jp
+g">
And a link to Google: <a href="http://www.google.com">Google</a>.
[download]

Arjen