inspired by your use of sum, the expected sum(1..n) = n*(n+1)/2, so take the found sum from the expected sum:

use List::Util qw(sum);
my @arr=(1..3,5..7); 
print +(1+@arr)*(2+@arr)/2 - sum(@arr);
[download]

update: clarification = use 1+ and 2+ here because an element is missing from @arr

That's a well oiled twist.

Everyone looking at arrays and comparisons when there was a mathematical solution hiding in plain sight.

... a mathematical solution hiding in plain sight.

Perhaps better to say "hiding in obscurity." From the OP (emphasis added):

... find a missing number if in a list of numbers. ...

...
my @arr = (19,17,22,23,24,15,16,25..35);
...

But the list in the code example clearly has more than one missing number. See also the (current) thread title. I think the goose chase, although wild, was justified.

---

Give a man a fish:  <%-{-{-{-<

Ahh, I stand corrected. It's funny how user requirements can hinge and swing wildly from a small misunderstanding.