I think you confused some people You want to know, how do you print a line to the file if the file doesn't exist already? I think the following does what you are asking, you could remove the new lines and make it one line if you wanted but I wouldn't recommend it.

unless -e $file {
   open( my $fh, '>', $file) or die "Failed to open '$file': $!";
   print $fh "Header";
}
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Update: If you use IO::All then you could do  'Header' > io('test.txt') unless -e 'test.txt';