Perhaps something like this:

my @prefix;
while (<>) {
  next if /^#/; # skip comments, apparently
  if (/^\s*(\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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-- Randal L. Schwartz, Perl hacker