So, this is either non-responsive (since I'm not using regexes) or one of those "hey, I never thought of that" situations. A cheater approach to this problem is to use 'split':

use strict;
use warnings;

my $input = "!Frequency[A][B][C]  ..some other text..";
$input    =~ s/^!//;  # dump that leading '!'
print "'$input'\n";

my @pieces = split(/[\]\[]+/, $input);

foreach my $piece (@pieces)
{
   print "PIECE '$piece'\n";
}
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