undef $/;    # This sets your input record separator
             # (i.e. $INPUT_RECORD_SEPARATOR if you 'use English')
             # to undef (usually it's \n -- it's what makes 'read' read in
             # a line at a time. You could even set this to "[SDGT]\n" if
             # you wanted, and read in a whole actual record at a time,
             # if that was going to be a consistent marker of the end of
             # a record
             
my $stream = <FILE>;   # Read the whole file into one scalar variable

my @records = split($stream, m{\[[^\x5d]+\]\n}/);
   # Split the stream into records, based on a regexp, if you could
   # figure out a regexp that would consistently mark the end of a record
   # This should match any [.*] marker at the end of a record

foreach (@records) {
    s{\n}{ }g;   # Remove line breaks
    # You could also do your matching and marking up in this loop
}