I'm unclear why line 9 passes a syntax check...

Unary  ~ is bitwise negation. See Symbolic Unary Operators in perlop. It compiles, but is probably not logically correct. E.g.:

>perl -wMstrict -le
"$_ = 'x';
 print scalar(~ /x/);
 print scalar(~ /Y/);
 print 'match' if ~ /x/;
 print 'match' if ~ /Y/;
"
4294967294
4294967295
match
match
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Update: Improved example code.