Another option would be to use meta programming, e.g. starting with

grammar g {
    rule TOP { <bang> | <at> }
    rule bang { 'bang' '!' [<-[!]>*] '!' }
}

g.^add_method('at', rx { 'at' [\s*]  '@' [<-[@]>*] '@' } );
# ... then do the above for each of the sigils
g.^compose;

say g.parse('bang !some text!');
say g.parse('at @some text@');
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