while (<>) {
  # examine the first line of the file
  # do the appropriate thing with it
  # (for example:)
  if (/^Expense Report/) { $type = 'Expense' }
  elsif (/^Invoice/) { $type = 'Invoice' }
  else { die ... }

  # Now seek the file
  seek(ARGV, 0, 0) or die "seek: $!";
  process_data($type);  # call the appropriate processor
  # the rest of the data in the file has been eaten
}

sub process_data {
  my ($type) = shift;
  # call process_invoice or process_expense_report
  # depending on $type 
}

sub process_invoice {
  while (<>) {
    # do something
    last if eof;
  }
}