The problem is that your ... ($my_xpath)[0] ... is being interpreted as  ... ($my_xpath)->[0] .... As in, the thing on the left is expected to be an arrayref, which it isn't.

You can use a slice to get the proper effect:

my $bar=($twig->get_xpath($my_xpath))[0]->text;
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That'll provide list context to the inner expression, and then extract just the 0'th element from that.