I'll happily explain, thanks for asking.
First off, in Perl the parens for a function call are not mandatory.
As such it is very useful for functions and variables to be visually difference, especially when you're using functions to generate parameters for other functions without any temp variables inbetween.
my W = qq_mult ConjugateQ, qv_mult RotationQ, Vector;
You can't tell at a glance what's going on and will need to look carefully. Adding sigils makes it quite clear:
my $W = qq_mult $ConjugateQ, qv_mult $RotationQ, $Vector;
Further, most languages have only one type of variable, a name for a single thing. Perl has multiple types of variables that behave differently. For example:
my $res = munge $one;
I see this and know that the function munge is passed one single variable. However, consider this:
my $res = munge @two;
If there wasn't the @ there, it would be easy to assume that munge gets exactly one argument. However the @ there alerts us to the fact that @two is an auto-flattening array, and munge will end up with anywhere between 0 and MAX_INT arguments passed to it.
Similarly with hashes:
my $res = munge @two;
They also auto-flatten, so they need to be marked as being different from scalars, but they also flatten in a very different way from array, in that they flatten into a list that alternates the keys and values. So they need to be marked differently from arrays.
Lastly, due to functions, variables, array and hashes having explicit sigils, they can be recognized by editors without any heavy analysis, enabling editors to mark these four types with different colors, which is extremely useful. Personally i don't see the sigils anymore, and instead just see the colors with which the types are highlighted.
(Bonus set: Actually C is often written with sigils too. I've often seen code where variables are prefixes with p_, s_, a_, i_, etc. They are not enforced by the language, but people often force themselves to use them. The downside: They are inconsistent from project to project and have to be relearned every time.)