I do not know why you think it does nothing:

$ cat foo.pl
#!/usr/bin/perl -w

use strict;
use warnings;

my $text = '<font color="blue"><i><b> <br>Some text</font>,  <br>an a'
+;
$text =~ s~<font color=("|')?((\w|\s)+)("|')?>(.*?)<\/font>~\[color=$2
+\]$5\[/color\]~ig; 
print $text . "\n";

$ ./foo.pl
[color=blue]<i><b> <br>Some text[/color],  <br>an a
$
[download]

That's not to say the regex couldn't be improved/simplified, but it certainly seems to have an effect as it stands.

Maybe $text actually has newlines in it and you need to add the /s modifier? Also, bracketing regex delimiters are more readable.

$text =~ s{...}{...}igs;
[download]

The script you posted is working perfectly fine. There must be a problem with the actual input you are using. That being said there's quite a lot you could do to improve the RegExp:

((\w|\s)+) is better written as a character class, e.g.: ([\w\s]+)

Are you sure this is what you want? According to W3C there are no HTML-colors with whitespace in their name.
Use \s*(\w+)\s* instead? Additionally you fail to match valid colors like #ff0000

("|')? .... ("|')? -> would match "abc'; better use a back-reference: (["']?) .... \1

<\/font> can be replaced with </font> since you are using s~~~ instead of s///

\[color=$2\]$5\[/color\] can be converted into [color=$2]$5\[/color] (must keep the escape after $5 or perl complains)

Additionally: I don't know where the input is coming from and how you much you can trust it. Your code 'fails open' - nothing happens in case it fails to match. I would suggest to convert it into 'fail closed' - complain loudly or even terminate in case a font tag can not be matched propely.

while (parse input) {
  if (a font tag is found)) {
    if (font tag matches expected pattern) {
      (replace font tag)
    }
    else {
      (complain loudly)
    }
  }
}

HTML::BBReverse - Perl module to convert HTML to BBCode and back.

If you post an actual line that is failing it might be helpful.

So this bit of code is a bit longer than I expected but now that I've done it I might as well post. Note it only replaces font tags that have a color attribute, everything else is left as is.

use HTML::Parser ();

my $parser = HTML::Parser->new(
    api_version  => 3,
    start_h      => [\&start_tag,    "tagname, attr, text"],
    text_h       => [\&text_content, "text"],
    end_h        => [\&end_tag,      "tagname, text"],
);

my ( $font, %colored );

sub start_tag {
    my ($tagname, $attr, $text) = @_;
    if ($tagname eq 'font') {
        $font++;
        if (my $color = $attr->{'color'}) {
            print "[color=$color]";
            $colored{$font}++;
            return;
        }
    }
    print $text;
    
}

sub text_content {
    my $text = shift;
    print $text;
}

sub end_tag {
    my ($tagname, $text) = @_;
    if ($tagname ne 'font') {
        print $text;
        return;
    }
    if ($colored{$font}) {
        print "[/color]";
    }
    else {
        print $text;
    }
    $font--;
}


my $html1 = q|<font color="blue"><i><b> <br>Some text</font>,  <br>an 
+a|;
$parser->parse($html1);
$parser->eof;

print "\n\n";

my $html2 = q|<font color="blue"><i><b> <font>breaker</font><br><font 
+color="#ff0000">Some</font> text</font>,  <br>an a|;
$parser->parse($html2);
$parser->eof;
[download]

[color=blue]<i><b> <br>Some text[/color],  <br>an a

[color=blue]<i><b> <font>breaker</font><br>[color=#ff0000]Some[/color]
+ text[/color],  <br>an a
[download]

"Not working" is not helpful. What output do you desire? What output are you getting? We can be much more helpful when there's less guessing involved!

Not working in that nothing changes with the input

Desired output from

<color=blue><i><b> <br>Some text</color>,  <br>an a

would be..

[color=blue]<i><b> <br>Some text[/color],  <br>an a

Sorry struggling to use this website, can't seem to edit my post. The input is given in my first post, ignore the input in my second post as it's incorrect. The expected output is correct.