I kinda doubt it's fast but it certainly is short.

sub compat {
    my ($f,$s) = @_;
    my $neg = "[^".join("",grep $_ ne "_", split//, $f)."]";
    $f =~ s/((.)\1*)/$1 eq "_" ? "$neg\{".length($2)."}":"[$1_]"/ge;
    return $s =~ $f;
}
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