split() supports this type of thing. From the docs:

If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter.

split(/([,-])/, "1-10,20", 3); produces the list value

    (1, '-', 10, ',', 20)

So...for your example "x := y + z;":

$_="x:= y + z;" ;
for (split(/(:=|\+|;)/)) {
    print "[$_]\n";
}
[download]

Produces:

[x]
[:=]
[ y ]
[+]
[ z]
[;]
[download]