If it needs to be a reference to a scalar (as it appears it does), then you want this:

my $processor = \`/aa/bin/get_pbind_cpus.ksh`;
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That's kind of odd, however. I'd look at the other usages to see if they really are passing in a scalar reference. I'm not saying it's impossible, but I find it strange since there isn't really any advantage to passing in a reference rather than the variable itself.