Somewhat related to the comment by Fletch above, you could use an ARRAYref on success (which would be empty for no results), and a scalar containing an error message on failure.

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 mes
+sage
    die $a;
}
elsif (@$a) {
    # if @$a is nonzero, there were results
    ...
}
else {
    # there weren't results
    print "No results found, sorry!"
}
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