You don't really want to join the pipe. Nor do you likely want "-n 3". You probably want "-n 1".

#!/usr/bin/perl
open PIPE, "/usr/local/bin/top -b -n 1 |" or die "Cannot open pipe";
my ($output) = grep /^CPU states:/, <PIPE>;
close PIPE or die "Running top failed";
print "$output\n";
exit(0);
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Hope that helps,