As the return value of this expression is returned and checked for truthness, it's not used in a void context.

Are you sure about that? If you end a package with just: 1; no warning is produced; but if you end it with just: 42;:

C:\test>perl -Mstrict -w
package x;

sub f{ 1 }

42;
^Z
Useless use of a constant in void context at - line 5.
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Although it is returned and checked at runtime; at the point of parsing it is in a void context.

And you can't avoid that by prefixing a return statement either:

C:\test>perl -Mstrict -w
package x;
sub f{ 1 }
return 42;
^Z
Can't return outside a subroutine at - line 3.
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I usually don't run modules, I use them. No warning is produced in such situations.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

True. But before we even start talking about unit tests, it would be nice to have

perl -wc Foo.pm
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at least come up with the "Foo.pm syntax ok" message and without raising any warnings.

Given

package Foo;
42;
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this would give "syntax ok" but also the "useless use of constant in void context" warning, whereas

package Foo;
1;
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would only give the "syntax ok" message.

Good point :)