To achieve that I use bit-wise operations to test the bits of elements of @rows, and when appropriate, to set the bits of the elements of @cols. The following is essentially identical to the code above, but expanded a little to hopefully clarify things a bit:

## The rows
my @rows =  ( 0x04, 0x04, 0x04, 0x04, 0x00, 0x00, 0x04, 0x00 );

## Initialise the columns
my @cols = (0) x 8;

## For each of the 8 columns
for my $col ( 0 .. 7 ){

    ## look at each of the rows in turn
    for my $row ( 0 .. 7 ) {  

        ## And if the $col bit of the $row row is set
        if( $rows[ $row ] & 1 << $col ) {
        
            ## Set the $row bit of the $col column
            $cols[ $col ] |= 1 << $row;
        }
    }
}
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