Which output/errors do you get and what do you miss?

edit

do you notice something here?

qr{  abc_p | abc) }xms;

update

I ran the code because this looked too obvious to be true, alas ...

Unmatched ) in regex; marked by <-- HERE in m/  abc_p | abc) <-- HERE 
+ / at d:/tmp/pm/suv0.pl line 10.
[download]

Perl even points out the correct line ...

... sigh ...

...downvoted, again.