Okay!

Now it's at least running your script, but something is not 100% with the output... Try:

#!/usr/bin/perl
use strict;
use warnings;
use CGI qw(:all);
print header;
print "hello!";
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And chmod 755 a.pl just to be sure.

If it still gives the error, run the script from the command line (./a.pl) and paste the output...

(We'll get there yet!)