You could use the sum function from List::Util:

    use List::Util qw/sum/;
    $header{"Number of Drills"} = sum
            map $_->count,
            map { values %{ $_->tool } } @drill_arr;
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I haven't benchmarked this so don't know how it compares to yours. I'd guess it'd be faster since yours has to build up a code ref then get rid of it, etc. And sum is implemented in C, so it's fast.

Update: okay, benchmarked them:

    timethese(-6, {
        'sum' => sub {
            my $n = sum
                    map $_->{count},
                    map { values %{ $_->{tool} } } @drill_arr;
        },

        'sub' => sub {
            my $n = &{ sub {
                my $ret;
                foreach my $l ( @drill_arr ) {
                    foreach ( values %{$l->{tool}} ) {
                        $ret += $_->{count};
                    }
                }
                return $ret;
            } };
        },
    });
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Results:

Benchmark: running sub, sum, each for at least 6 CPU seconds...
       sub:  7 wallclock secs ( 6.00 usr +  0.00 sys =  6.00 CPU) @ 15
+883.17/s (n=95299)
       sum:  6 wallclock secs ( 6.02 usr +  0.00 sys =  6.02 CPU) @ 40
+993.85/s (n=246783)
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So using sum is lots faster, apparently.

Note however that I'm not using method calls; I just set up a dummy array holding hashes of hashes that I'm using.