So if # actually have higher precedence in regexp with /x modifier it is a bug, because it does not work as stated.

True. You can use perlbug to file a report.

If # had higher precedence than \Q..\E, string was just trimmed after the # character.

False. "#" doesn't trim anything.

>perl -e"print qr/4d#sd/x"
(?x-ism:4d#sd
)
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