According to the new source::encoding pragma's documentation,

use source::encoding 'ascii' is automatically enabled within the lexical scope of a use v5.41.0 or higher.

To me, this means that

use v5.42;

is equivalent to

use v5.42;
use source::encoding "ascii";

But that doesn't seem to be the case.

$ perl -e'use v5.42; my $x = "é";'

$ perl -e'use v5.42; use source::encoding "ascii"; my $x = "é";'
Use of non-ASCII character 0xC3 illegal when 'use source::encoding "ascii"' is in effect at -e line 1, near ""ascii";"
Execution of -e aborted due to compilation errors.

Bug or misunderstanding?