Found the issue myself - it was in the perl function. It was getting an array, whereas it needed to expect a reference to array. The wsdl =begin =end definitions stay the same though:

=begin WSDL
    _IN smis @int session message id's to get data for
    _RETURN @SmiData smi data for each existing modality
=end WSDL    
=cut


sub getSmisData
{
    my $class = shift;
    
    my ($smis) = @_; # not @smis
    
    my $smiData = [];
        
    my $dbh = open_db();
    
    if ($dbh)
    {
        # do something    
    }
    else
    {
        # do something else
    }
    
    return ($smiData);
}
[download]

Thank you all for trying.