print header;
while ( <FILE> ) {
    # ... 
}

##</code><code>##

while ( <FILE> ) { 
    # ...
  print header unless $already_printed_header++;
    # ...
}

##</code><code>##

use CGI qw/ -unique_headers /;

while ( <FILE> ) { 
    # ...
  print header;
    # ...
}