Change the following line:

use CGI;
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...to...

use CGI qw/:standard/;
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The problem is that the header isn't getting printed. It's not getting printed because you didn't ask it to be imported. The qw/:standard/; does that for you.

If you check the error logs you would also see (in the error logs) why the script failed. Strictures will complain:

Unquoted string "header" may clash with future reserved word at mytest
+.pl line 8.
Name "main::header" used only once: possible typo at mytest.pl line 8.
print() on unopened filehandle header at mytest.pl line 8.
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