The -p switch instructs perl to wrap your code with:

LINE: while (defined($_ = <ARGV>)) {
    # Your code here
}
continue {
    die "-p destination: $!\n" unless print $_;
}
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This transformation is so direct that you can use the token }{ to your advantage. My original code now becomes:

LINE: while (defined($_ = <ARGV>)) {
    $\ += $_;
} 
{ }
continue {
    die "-p destination: $!\n" unless print $_;
}
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The while loop now focuses on summing the input (converting each line to a number in Perl's fashion, i.e. stopping after any non-number chars). The continue is now attached to an empty run-once block. So, the printing of $_ only occurs once, and after the input has been exhausted, and $_ is now undef. The print statement prints the empty $_, but also always prints $\ (the line terminator), which currently has our sum.

So, this is a sum command. Can be used with wc ... | perl ... or du ... |, etc.

I never code like this in production code, of course! ;-)