I think you may have misunderstood the intent of the original
subroutine. Given an integer, it should produce a list of three
values: red, green, and blue. It does not produce anything like
"0000FF"; rather, given 0x0000FF (note the difference, BTW),
it should produce the list (0, 0, 255). Which it does.
Where you say that it is returning 00255; it actually is not. It's
returning a list containing (0, 0, 255), and when you print that
out, it's getting automatically joined on the empty string, and
so you see 00255.
But the actual usage is like:
my($red, $green, $blue) = hexa_triplet(0x0000FF);
And then the $red, $green, and $blue would be passed into
GD, since that's what the OP needed this for.