You're modifying the array you're iterating over. Don't do that.

Personally, whenever doing a nested iteration over two arrays, I try to convert at least one the inner array into a hash and do a hash lookup into the inner array. This is usually faster and makes for simpler and shorter code too.

Of course, the challenge is to find a good common key to use in the hash. In your case, it seems that you only want to act upon a file in @return2 if it has the same size as a file in @remoteFilelist, and if its name starts with the same letters as that file. Your comment indicates that all local files are named myfile.txt_date. So the key could be myfile.txt|localsize:

# First, build the hash of names+size of all the local files
my %local_file;
for my $filename (@return2) {
    $filename =~ /^(.*)_(20\d{6})$/
        or die "Couldn't find date part in filename '$filename'";
    my $remote_name = $1;
    my $local_size = -s $filename;
    my $key = join "|", $remote_name, $local_size;
    $local_file{ $key } = $filename;
};

for my $remote_file (@remoteFilelist) {
    my $remote_size = -s $remote_file;
    my $key = join "|", $remote_name, $remote_size;
    if (! exists $local_file{ $key }) {
        print "$remote_file does not exist locally, or exists locally 
+but with the wrong size.\n";
    } else {
        print "$remote_file exists locally as $local_file{ $key }\n";
    }
};
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