my $fname = "while_multi_cond.dat";
my $s1 = "";

open( FP,  "<", $fname ) or die "Can't open $fname: $!";

while(<FP>) {
  if (/^_start/) {

#   This works as expected
#   while(<FP> ) {
#     if (/^_stop/) { last }

# I'm trying to replace it with this:  

     while( (<FP>) && (!/^_stop\n/) ) {    

# but seems to break the functionality of $_

      $s1 .= $_;
    }
  }
  if ($_) { print("$_") }
}

print("\$s1= \n<$s1>");