perl -ne "$. ==1 and print;" filename.txt
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perl -e "print scalar <>;" filename.txt
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perl -ne "print and last;" filename.txt
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perl -ne "last; END{print}" filename.txt
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perl -ne "die $_;" filename.txt
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perl -pe "$. > 1 and last;" filename.txt
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Some will be more efficient than others.

The point is that if all you need to print is the first line, you should probably terminate reading the file once you've read the first line.

perl -ne 'print && close ARGV' filename.txt
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... it even works on stdin.

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';

Some great solutions. How about:

head -1
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Assuming a *nix platform.