perl -pe 's/\t/\\t/g' infile > outfile

##</code><code>##

my %mapping = ( # or whatever you like to display
  "\t" => '\t',
  "\n" => '\n',
  " "  => '_',
);

my $pattern = qr/([@{[join '', keys %mapping]}])/;

while ( <> ) {
  s/$pattern/$mapping{$1}/g;
  print "$_\n"; # you still want to start a new line
}