http://www.perlmonks.org?node_id=976729


in reply to I don't understand a piece of this code

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";

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

In Section Seekers of Perl Wisdom