my $str = '[id=4590]blahblah[/id]';

$str =~ m/^\[id=(\d+)\](.+)\[/;

print '<a href="/cgi-bin/coolcode.pl?id=' . "$1>$2</a>\n";

##</code><code>##

$str =~ s{^\[id=(\d+)\](.+)\[/id]}{<a href="/cgi-bin/coolcode.pl?id=$1>$2</a>};
print "$str\n";