For specifically the two tags shown (ignoring the stray backslash in the example), I would use:

  $text =~ s/\[(b|glow)\]/<$1>/g;
[download]

But for a general case, if one even exists, I would use:

  $text =~ s/\[([^\]]+)\]/<$1>/g;
[download]

But this is more dangerous, as those square brackets are in my opinion likely to occur in text.