dws says :

  my $attr = shift;
  my @found = { grep /^on[A-Z]/ } keys %$attr;
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I thought about doing this, but I wanted to stick to known events. If that's not a concern, then your grep is acceptable as well.
I originally started out with an inclusive checker, but the performance was pretty awful as I was checking that each tag had acceptable attributes. (An alt attribute is ok for an img tag, but not a p tag, fr instance).
Another potential speed boost (on closer examination) would come from making all @events lowercase, and getting rid of the i switch in the current grep, as HTML::Parser passes parameters to its handlers in all lowercase.