Correction:

use strict;
open (OUT,">top.log") or die "cannot create: $!\n";
for (;;){
    my $top = `top n 1`;
    print OUT $top;
    sleep 10;

}
close OUT;
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Sorry I was actually abbreviating my original script from memory. The script writes and executes top just fine when I don't run it as a background job. The explanation that top does not run as a background job is very helpful. Thank you.