None of your examples had two keywords before a curly brace, so I didn't support that.

Just a hint for the future: specifications help, and examples help too.

So, another whack at this:

my @prefix; 
while (<>) { 
  next if /^#/; # skip comments, apparently 
  if (/^\s*(.*?)\s*\{\s*$/) { 
    push @prefix, $1; 
  } elsif (/^\s*\}\s*$/) { 
    pop @prefix; # no error checking 
  } elsif (/^\s*(.*)/) { 
    print "@prefix " if @prefix; 
    print $1, "\n"; 
  } else { die "internal error on $_"; } 
}
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But the real question I gotta ask is, "did you learn anything, or did I just do your homework for you?"

-- Randal L. Schwartz, Perl hacker