Well, actually this all wrong. chomp certainly doesnt belong on the right side of a =~ operator. The way to write this is to use the idiom that dragonchild outlined:

while (<$in>) {
    chomp(my $line=$_);
    next unless length $line;
    print;
}

__END__
# please excuse this stuff here, just doing some pmdev testing with th
+is node :-)
# it will be removed soon.

# No indent
 # single space
  # two spaces
   # three spaces
    # four spaces
# No indent
 # single space
  # two spaces
   # three spaces
    # four spaces
[download]

:-)