You need to escape the ['s and ]'s.  qr"..." and \Q and \E are useful for that. Consider:

my $haystack = 'lets test[1] this thing $string';
my @tests = ('test[1]', '\Etest[1]\Q', 'test\[1\]');
@tests = map {$_, qr"\Q$_\E"} @tests;

print "Check $_:" . ($haystack =~ /($_)/ ? "got $1\n" : "no match\n") 
+for @tests;
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which results in:

Check test[1]:no match
Check (?^:test\[1\]):got test[1]
Check \Etest[1]\Q:no match
Check (?^:\\Etest\[1\]\\Q):no match
Check test\[1\]:got test[1]
Check (?^:test\\\[1\\\]):no match
[download]