Don't do CGI decoding yourself, use CGI.pm or CGI::Lite:

#!/usr/bin/perl -w
use strict;
use CGI;

my $q = CGI->new();
my @pairs = $q->param('Blah[]');
for (@pairs) {
    print "Blah: $_<br />\n";
};
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Update: It is ->param('Blah[]'), not ->param('Blah')