my %pair = (
     'foo'   => 'bar',
     'jack'  => 'daniels',
     'Tom'   => 'Jerry',
     'BEGIN' => 'END',
);

my %data;
my $tail;
my $type;
while (<FILE>)
{
      chomp;
      s/\s+$//;    # Clean up invisible stuff

      if ($pair{$_})
      {
            $type = $_;
            $tail = $pair{$_};
      }
      elsif ($_ eq $tail)
      {
            undef $type;
      }

      push (@{$data{$type}}, "$_\n");
}