my @list = ( -4 .. 6 );
print map((1)x$_,@list) - map((1)x-$_,@list);
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Wow. That's a exactly (and I mean, exactly) the first idea I had to solve the negative numbers problem. I didn't post it, though, because it doesn't handle decimal numbers.

sub sum { eval 'pop(@_)+' x ($#_ + 1) . '0'; }

@list = (-4, 15, -5, 2.5);
print sum @list;            # 8.5, as expected
print sum -4, 15, -5, 2.5;  # Works the same
print sum;                  # 0 (not undef)
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