Well, Parham and Juerd summed up the most common ways to do it. But for the sake of tmtowtdi, here is YAW (yet another way ;)

Rewrite the return portion of someonesFunc() to always return a scalar, either a string/number or an array refrence.

my $tmp = someonesFunc();
if ($ref $tmp eq 'ARRAY') {
    # use @$tmp as an array
} else {
    # use $tmp as a scalar
}
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