Thank you very much to everybody. Both solutions work very well, that with grep and that with map. I have tried to make the map one work with foreach, but I gave up, because of the inability of erasing the empty references without leaving the slots undefined. Again, thank you. Best regard Luca

sub prune_with_map 
{
    my $aref = shift;
    @{ $aref } = map 
    { 
        if( ref( $_ ) ) 
        {
            if( @{ $_ } ) 
            {
                prune_with_map( $_ ); 
            }
            else 
            { 
                ();
            }        
        }
        else 
        {  
            $_;
        }
    } @{ $aref };
    return ( $aref ); 
}

sub prune_with_foreach
{ 
    my $aref = shift;
    my $count = 0;
    foreach ( @$aref )
    { 
        if( ref( $_ ) ) 
        {
            if( @{ $_ } ) 
            {
                prune_with_foreach( $_ ); 
            }
            else 
            { 
                delete $aref->[ $count ];
            }        
        }
        else 
        {  
            $_;
        }
        $count++;
    }
    return ( $aref );
}
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