I have a snippet of code I'm working on, I'm decent with perl, but as I've always had to learn in a hurry, my all-around knowledge is lacking. What I have here is a matrix of data, and I need to change the first field in each row into undef so that I can continue with the rest of the script

sub indextime{
    open FILE, "<p2cash.prn";
    open OUTP, ">p2cashmast.txt";
    select STDOUT;
    undef (@matrix);
    while (<FILE>){
    
    push @matrix, [split];
    }

    $storenum = $matrix[0][0];
    $date = $matrix[0][1];
#approx line 100
    
    print "date $date \n ";
    print "store$storenum \n ";
    #@matrix[0] = undef;
    #@matrix[1] = undef;
    #@matrix[2] = undef;
    #@matrix[3] = undef;

    
    select OUTP;
    for $i (6.. ($#matrix-1)){
    #$matrix[i][0] = $storenum;
    $matrix[i][0] = undef;
    print "@{$matrix[$i]} $storenum $date\n";
    
    }
    
    
}
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I tried the $matrix[i][0] = undef, but that didn't do it.