push @{$hash{A1}{B2}{C1}}, $item;

##</code><code>##

$ perl ttt3.pl
Can't use string ("") as a HASH ref while "strict refs" in use at ttt3.pl line 7.
* /W/P/PM/ monitor_saints;  check TODO_LIST

$ cat ttt3.pl
#!/usr/bin/perl
use strict; use warnings;
use Data::Dumper;

my %hash;
$hash{A1}{B2} = "";
if (exists $hash{A1}{B2}{C1}) {
    print "Baz!\n";
}