I came up with a one-line pop solution:

my $max = pop @{@vals=sort @vals; \@vals};
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It's kind of cheating in the "one-line" department, and I don't recommend it as clear programming, but it illustrates that "a BLOCK returning a reference" really means a BLOCK, not just an expression in curlies. Obviously the clear way to write it would be

@vals = sort @vals;
my $max = pop @vals;
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