As chromatic already explained, := creates an alias, while = puts a value into a container.

What's more is that = also enforces a context, so for example assigning to an array variable causes it to upgrade the right-hand side to an array. Binding doesn't:

$ perl6 -e 'my @a = 1, 2, 3; push @a, 4; say ~@a'
1 2 3 4
$ perl6 -e 'my @a := 1, 2, 3; push @a, 4; say ~@a'
Method '!fill' not found for invocant of class 'Int'
  in 'List::push' at line 2617:CORE.setting
  in main program body at line 1
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The error message isn't good, but what happened is that I tried to push onto a list of values (not to an Array, because the binding replaced the array variable with a List). A List is read-only, so it blew up.

Assigning to arrays is also eager, whereas binding doesn't evaluate lists at all:

my @a = gather for 1..3 {
    .say; take $_;
}
say "printing now...";
say ~@a;

# produces

1
2
3
printing now...
1 2 3
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But with binding, you get lazy evaluation of the gather/take construct:

my @a := gather for 1..3 {
    .say; take $_;
}
say "printing now...";
say ~@a;

# produces
printing now...
1
2
3
1 2 3
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Only on accessing list items are they evaluated as needed.