my $file;
if(case1) {
  $file = 'this.html';
} elsif(case2) {
  $file = 'that.html';
} else {
  $file = 'error.html';
}

print "Content-Type: text/html\n\n";
{
  open(HTML, $file) or die $!;
  local $/; # this will make <HTML> just return the 
            # whole file instead of line by line
  print <HTML>;
  close HTML;
}