You don't need to check (you could use Scalar::Util if you did, or eval)

$_[0] ||= "anonymous";  # <<<<<<<<<<<<<<<<<<< wrong wrong wrong
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The intent of your function is to modify $body (the 3rd argument), so restrict yourself to modifying only $body. It is (imho) a better idea to do it like

sub roy {
    my( $f, $b, $modifythisREF ) = @_;
    $$modifythisREF =~ s/yoda/yoda/g;
    return;
}

roy( "", "", \$body );

...
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