why the $ARGV[0] in while loop cannot be accessed?

Because the diamond operator (when used with the null filehandle <>) consumes the contents of @ARGV.

See the section of perlop I/O-Operators that starts with the sentence:

The null filehandle <> is special:
for the details.
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In reply to Re: @ARGV in while loop by BrowserUk
in thread @ARGV in while loop by lightoverhead

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