# Calculate screen
for my $yi (0..$ROWS-1) {
    my $y = $yi * $yfact;
    for my $xi (0..$COLS-1) {
        ...
    }
}
[download]

Should be something like...

# Calculate screen
for (my $yi = 0; $yi < $ROWS; $yi++) {
    my $y = $yi * $yfact;
    for (my $xi = 0; $xi < $COLS; $xi++) {
        ...
    }
}
[download]