Well it is pretty simple.

The /x lets you have whitespace and comments in your RE.

(?:blah) means match 'blah' but don't capture into $1. This is useful if you want to group using () but not capture. In this case it is looking for * worth of ([\x00-\x7F]|[\x81-\x9F\xE0-\xFC][\x40-\x7E\x80-\xFC]|[\xA0-\xDF]) where the star means 0 or more grabbing as many as possible. The ? after the * is a modifier that says don't grab as many, grab as few as possible (non greedy)

The | is alternation ie this|or|that|or|other.

[A-Z] is a character class that will match the letters A-Z in that case.

The /s is totally redundant as it lets the . regex metachar match anything (normally mathces everything except \n) but as there are no . metachars it does zip.

Finally the /o is a promise to Perl that $encoding and $search will not change during the entire runtime of the script so that Perl can interpolate these scalars, compile the RE and then not recompile ever again. If you change the interpolated scalars after the first time the RE runs Perl will not notice with /o