Hint: When in doubt use B::Deparse with option -p for parenthesis

d:\exp\version>perl -MO=Deparse,-p -E"$a=A;$b=B;print 'Matches!' if $a
+.$b =~ /C/"
use feature 'current_sub', 'evalbytes', 'fc', 'postderef_qq', 'say', '
+state', 'switch', 'unicode_strings', 'unicode_eval';
($a = 'A');
($b = 'B');
(($a . ($b =~ /C/u)) and print('Matches!'));
-e syntax OK
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