use constant BLOCKLENGTH => 32768; # > length of any tag
$\ = \BLOCKLENGTH;
my $buffer = '';
while (!eof()) {
   $buffer .= <>;
   s/tag1/\n/g;
   s/tag2/\t/g;
   # leave BLOCKLENGTH chars in $buffer,
   # print however much comes before that
   print substr($buffer, 0, -BLOCKLENGTH, '');
};
# print whatever's left
print $buffer;