Like I already said, $ARGV[0] always evaluates to a scalar. As long as you keep using $ARGV[0], you'll always get a scalar. If you want to pass zero when @ARGV is empty, you'll need to pass something different when @ARGV is empty. In general, you'd do 

p1( @ARGV ? $ARGV[0] : () );

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In this case, the following suffices: 

p1(@ARGV);

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In reply to Re^3: Question: Is undef a valid argument? by ikegami
in thread Question: Is undef a valid argument? by jujiro_eb

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