As "h" is true that expression will always be true.

To be pedantic, it is not because "h" is true that the expression is true, otherwise

would evaulate to false, which it doesn't.

Rather, if my reading of perlop is correct, it is because the assignment operator returns true (or more strictly, the number of elements assigned to @ARGV):

my $return = @ARGV = ( 1, 2, 3 );
print $return;
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prints 3.

Update: fixed typos in text.