sub requestDB {
    ...
    if ($some_error) {
        return "This error occurred: $DBI::errstr";
    }

    return \@results; # reference to results array
}

my $a = requestDB(...);
if ( ! ref $a ) {
    # if $a is not a reference, it's a scalar that holds the error message
    die $a;
}
elsif (@$a) {
    # if @$a is nonzero, there were results
    ...
}
else {
    # there weren't results
    print "No results found, sorry!"
}