/usr/share/perl5/Mail/IMAPClient/Thread.grammar is in libmail-imapclient-perl 3.39-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | # Atoms:
NUMBER: /\d+/
# Rules:
threadmember:	NUMBER    { $return = $item{NUMBER} ; }	|
		thread 	  { $return = $item{thread} ; } 
thread:	"(" threadmember(s) ")"
	{
		$return = $item{'threadmember(s)'}||undef;	
	}
# Start:
start: /^\* THREAD /i thread(s?) {
	$return=$item{'thread(s?)'}||undef;
}
 |