my $RootDirectoryCanidate= shift() or return undef();
my $RootDirectoryCanidate= shift() || return undef();
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The question is which one should I use to accomplish my requirement?

Neither. The both return undef if no argument is passed, or if the first argument is 0, "0", or the empty string. What I would do is:

sub method {
    return if @_ && !defined $_ [0];
    my $root_directory_candidate = shift;
    ...
}
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Now, if you want to return right away if nothing is passed as well, make the first line in the method:

    return unless defined $_ [0];
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Note that I didn't do "return undef". If you do that, and you call the sub in list context, it will return a *true* value (a one element list). But a return without arguments will return an empty list in list context, and undef in scalar context.

Abigail