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

sub cpu_stat {
   open(STAT_FILE, "< /proc/stat") or warn "/proc/stat: $!";
   my $line = <STAT_FILE>;
   close(STAT_FILE);
   $line =~ /^cpu\s+(\d+) (\d+) (\d+) (\d+)/;
}

my @before = cpu_stat();   sleep 1;
my @after  = cpu_stat();

my $ticks_past = sum(@after) - sum(@before);
my $scale = 100 / $ticks_past if $ticks_past;  # divide by zero error

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

##</code><code>##

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