use strict;
use warnings;

my @stack;
while(<DATA>) {
  if( s|^(<(\d+)>.*)(</\2>\n)|$1\n| ) {
    print pop @stack while @stack and substr( $stack[-1], 2, -2 ) >= $2;
    push @stack, $3;
  }
  print;
}
print reverse @stack;

__DATA__
<1>one</1>
data data data data
<2>two</2>
data data data data
<3>three</3>
data data data data
<3>blah</3>
data data data data
<2>blah</2>
data data data data
<4>zoom</4>
data data data data
<4>zoom</4>
data data data data
<1>data</1>
data data data data