open my $fh,'<', $filename or die $!;

my $flag;
while( <$fh> ){

   unless( $flag ){
      next unless /^start_mark/;
      $flag = 1;
   }
   elsif( /^end_mark/ ){
      
      $flag = 0;
   }
}
continue {
   print if $flag
}

close $fh;