This works, but why isn't this function built in somewhere? It seems most responders assume that this is perl's default behavior.

Because that function is trivial to write and it isn't the only or even the obviously right choice.

sub first_or_all {  wantarray ? @_ : $_[0] }
sub last_or_all {   wantarray ? @_ : $_[-1] }
sub list_or_aref {  wantarray ? @_ : \@_ }
sub aref_or_list {  wantarray ? @{$_[0]} : $_[0] }
sub pairs_or_href { wantarray ? @_ : {@_} }
sub href_or_pairs { wantarray ? %{$_[0]} : $_[0] }
sub all_or_concat { wantarray ? @_ : join $,, @_ }
sub all_or_die {
    return @_
        if  wantarray;
    return $_[0]
        if  @_ <= 1;
    require Carp;
    local $Carp::level = 2;
    my $sub = ( caller(1) )[3];
    Carp::croak(
        "Don't call $sub() like that in a scalar context" );
}
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(to list just a few off the top of my head that I've heard people argue are wise choices)

- tye