The code is doing what you've understood:

$what = "[box]";
foreach (qw(in[box] out[box] white[sox])) { #Assign to $_, one by one,
+ each value in parenthesis
if (/\Q$what\E/) { 
print "$_ matched!\n";
[download]

\Q and \E disable special meaning of all metachars in between them. See perlre