Your specification seemed to say "match [error] followed by zero or more things contained in square brackets" so I was a bit surprised when your re did not contain the string "error" :) Here is a version that looks for "[error]", not sure if it is any better-

    
 s/(            # begin capture
     \[error\]  # "error" in square brackets :)
     \s*        # optional whitespace
     (?:        # non capture
       \[       # literal [
       [^\]]+   # 1 or more non "]"
       \]       # literal ]
      )*        # end non capture 
    )           # end capture
   /$1<br>/x;
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--
my $chainsaw = 'Perl';