But how would I rewrite my tests, then — which actually want to operate on the captured value?

This is Perl, so there's more than one way to do it, eg

my %sigils =
    bang   => '!', at      => '@', hash  => '#',
    dollar => '$', percent => '%', caret => '^',
    and    => '&', star    => '*', zero  => '0';

my regex line {
    ^^ :my $s; (\w+) <?{ $s = %sigils{$0} }> \h+ $s (\N*) $s $$
    { make ~$0 => ~$1 }
}

sub parse($_) { map *.<line>.made, m:g/<line>/ }

say parse "star *foo*\nat @bar\nat @baz@";
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will generate a sequence of pairs with the key being the name of the sigil and the value the enclosed text.