The Synopsis for Readonly 1.03 and 2.00 do in fact show this form:

# Alternate form (Perl 5.8 and later)
Readonly    $sca => $initial_value;
Readonly my $sca => $initial_value;
Readonly    @arr => @values;
Readonly my @arr => @values;
Readonly    %has => (key => value, key => value, ...);
Readonly my %has => (key => value, key => value, ...);
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