If you're worried about looks you could define a prototyped sub which takes a block, key, and the hash to which to add.

sub add_action (&\%$) {
  my( $code, $h, $key ) = @_;
  $h->{$key} = $code;
}

my %dispatch;
add_action { do_something_with_foo } %dispatch => qr/foo/;
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As an aside, you might want to look at Tie::RefHash which will keep the compiled regexen as keys rather than stringifying them (they'll get recompiled when you use them /$re/, but it's kinda inefficient).