The oct function doesn't "convert" a number to octal. It parses a string which contains the octal representation of a number (and in fact also quietly supports binary and hexadecimal notations too) and returns it as an integer. It essentially acts like eval, but supports an incredibly tiny subset of Perl.

  $number = oct('0644');
  $number = eval('0644');  # same
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In your particular case, you could just do this:

my $n = ($x<<24) + ($y<<16) + ($z<<8) + 0x40;
$rect->set('fill-color-rgba' => $n);
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Which will probably run quite a lot faster as it's pretty basic integer maths - just bit shifting and addition.

An alternative (no faster nor slower) than the above is:

my $n = ($x<<030) + ($y<<020) + ($z<<010) + 0x40;
$rect->set('fill-color-rgba' => $n);
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Why would you want to do it that way? Because if you're familiar with octal it might help you see what's going on a little better.

Or you can use multiplication instead of bit shifting. This might be a little slower, but if you're unfamiliar with bitwise operations, might be a little clearer:

my $n = ($x * 2**24) + ($y * 2**16) + ($z * 2**8) + 0x40;
$rect->set('fill-color-rgba' => $n);
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