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use List::Util 'sum';   # might be overkill

...
# Arguably, this is less readable than your code
printf("user: %d%%, nice: %d%%, total: %d%%, idle: %d%%\n",
   map { ($after[$_] - $before[$_]) * $scale } 0 .. 3);

my %hash;
@hash{qw/ user nice total idle /} = cpu_stat();
print "nice: $hash{nice}\n";