A subroutine may only know whether its caller wants a SCALAR or an ARRAY. The wantarray function handles this.

sub test {
  if (wantarray){
    return 'One String';
  } else {
    return ('A', 'List', 'of', 'Strings');
  }
}
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There is no built-in way to tell if a subroutine's caller wants a hash, but davorg has posted a sample of how you would do this (see his answer).

The DBI module uses a similar technique (non-relevant code removed for brevity):

sub fetchall_arrayref {
    my $slice= shift || [];
    my $mode = ref $slice;
    if ($mode eq 'ARRAY') {
        push @rows, [ @$row ] while($row = $sth->fetch);
    }
    elsif ($mode eq 'HASH') {
        push @rows, $row while ($row = $sth->fetchrow_hashref);
    }
    else { 
        Carp::croak("fetchall_arrayref($mode) invalid") 
    }
    return \@rows;
}
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If you call fetchall_arrayref with an array reference (or no reference), it will return array references. If you call it with a hash reference, it will return hash references.

  $sth->fetchall_arrayref( [] ) # returns Array references
  $sth->fetchall_arrayref( {} ) # returns Hash  references
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Cool stuff!

Russ