use strict;
use warnings;

my $debug_flag = 0;                     # not debugging by default

$debug_flag = $ARGV[0] if @ARGV == 1;
$debug_flag = ( $debug_flag eq lc ( '-debug' ) )? 1 : 0;

if ( $debug_flag )
{
   open DEBUG_FILE, ">debug.out" 
        or die "Cannot open debug file for writing: $!";
}

my $output;
# do stuff to get the output
$output = 'This is a test' . "\n";

print $output;
print DEBUG_FILE $output if $debug_flag;

# etc...

##</code><code>##

# warning, not tested

my $out_fh;

if ( $debug_flag )
{
	open FILE, ">debug.out" or die $!;
	$out_fh = *FILE;	
}
else
{
	$out_fh = *STDOUT;
}

# and then always do:

print $out_fh $output;	# it will go to the right place.