There are two easy ways to do this. One is splice, as others have shown:

splice @fred, 2, 0, "hello";
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the other is

@fred[2 .. @fred] = ("hello", @fred[2 .. @fred - 1]);
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which has the advantage that you don't have to remember the way splice interprets its arguments.

Update: D'oh, there's one more way I've forgot, although I've done something similar in Re: finding top 10 largest files:

@fred = (@fred[0 .. 2 - 1], "hello", @fred[2 .. @fred - 1]);
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