That's because
Build --verbose anything
is treated as
Build --verbose
Demo:
Readonly-2.05>perl Build.PL
Creating new 'Build' script for 'Readonly' version '2.05'
Readonly-2.05>Build --verbose install
cp lib/Readonly.pm blib\lib\Readonly.pm
mkdir blib\arch
Readonly-2.05>Build --verbose install
Skip blib\lib\Readonly.pm (unchanged)
Readonly-2.05>Build clean
Readonly-2.05>perl Build.PL
Creating new 'Build' script for 'Readonly' version '2.05'
Readonly-2.05>Build --verbose
cp lib/Readonly.pm blib\lib\Readonly.pm
mkdir blib\arch
Readonly-2.05>Build --verbose
Skip blib\lib\Readonly.pm (unchanged)
Readonly-2.05>Build clean
Readonly-2.05>perl Build.PL
Creating new 'Build' script for 'Readonly' version '2.05'
Readonly-2.05>Build
cp lib/Readonly.pm blib\lib\Readonly.pm
Readonly-2.05>Build
Skip blib\lib\Readonly.pm (unchanged)
One should use Build install --verbose
Readonly-2.05>Build install --verbose
Skipping C:\progs\SP5038~1\perl\site\lib\Readonly.pm (unchanged)
Writing C:\progs\SP5038~1\perl\site\lib\auto\Readonly\.packlist
It's skipping the install because it's already installed from an earlier attempt, but you can see it's installing this time.