Update: nemp's post above this got it first and got it right.

It should be $" not $, Below corrected.

@a = ( 'a', 'x', 'z' ); # simplified array
{ # restrict scope of changes to $,
    local $"=''; # change from default space to null string
    while(<>)
    {
        print if(/^[@a]{3}$/);
    }
}
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