Really, I too was surprised. Changing sub test to

sub test {
    my @rv = $_[0] =~ /^([0-9]+)$/;
    print "<<@rv>>\n";
    return @rv[ 0 .. $#rv ];
}
[download]

produced

<<>>
[[]
<<>>
[]
<<123>>
[123]
<<123>>
[123]
[download]

changing sub test to

sub test {
    my @rv = $_[0] =~ /^([0-9]+)$/;
    print "<<@rv>>\n";
    return wantarray? @rv: $rv[-1];
}
[download]

produces

michael$ perl t.pl
<<>>
Use of uninitialized value in print at t.pl line 5.
[]
<<>>
[]
<<123>>
[123]
<<123>>
[123]
[download]

Note the warning. (And I suspect there are some monks who are now saying I told you so;-)