I agree that either @sorted_original_cellnames or @sorted_top_cellnames must have undefined elements in it. I tried the following code:

#! /usr/bin/perl -w
use strict;
my @a = (1,2,3,4,5);
my @b = (2,4,6,8);
my %c;
@c{@a} = undef;
my @bb = grep { ! exists $c{$_} } @b;     
my %d;
@d{@b} = undef;
my @aa = grep { ! exists $d{$_} } @a;
print "@aa\n";
print "@bb\n";
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and I got the following output with no warnings:

1 3 5
6 8
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If I add an undef to @a or @b in my example, I get the warnings you describe.