my $code = <<EOT;
[head]
Head text...

[body]
Body text...

[something else]
more text..
EOT

$code =~ s/[\n ]+?(?=\S)//sg; #remove all newlines but last

formatCode($code);

sub formatCode {
    my $str = shift;

    $str =~ s{ \[ ([^\]]+) \] ([^\]]*) (?=\[|\n)}
             {<div class="$1">$2</div>}igx;

    print $str;
}

##</code><code>##

my $code = <<EOT;
[head]
Head text...

[body]
Body text...

[something else]
more text..
EOT

$prev_header = 0; # takes care of closing divs
for (split /\n/, $code) {               # No need to remove newlines
    if (/^\s*$/) { next }               # Skip blank lines

    elsif ( /^\[ ([^\]]+) \]/x ) {      # If line is a header...
        print "</div>" if $prev_header; #   close prev div, if there was one...
        print qq(<div class="$1">);     #   print current div...
        $prev_header = 1;               #   and set current div flag.
    }

    else { print "$1 " }                # Simply print non-header lines
}
print "</div>"; # You always have to close last div, so just do it here

##</code><code>##

open, $FH, "<whatever.txt";
for (<$FH>) {
    chomp;
    ...
}