That's because

Build --verbose anything
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is treated as

Build --verbose
[download]

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)
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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
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It's skipping the install because it's already installed from an earlier attempt, but you can see it's installing this time.