open( my $testfile, "+>> $file" ) $!\n";
if ( -z "$testfile" ) ---> checking for empty file
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This does not even compile. Post the actual code.

Stringifying the file handle in $testfile will give you something like GLOB(0xb03cb8), NOT the filename. This test will always fail.

To test if the file is empty, you do not need the -z test at all. You open the file for appending, so the file position is set to the end of the file. This is at position 0 only if the file is empty. tell will tell you the file position, or -1 on error. So, to repair that test:

open my $testfile,'+>>',$file or die "Can't open '$file': $!";
my $pos=tell $testfile;
($pos!=-1) or die "tell failed for '$file': $!";
if ($pos==0) {
    # file is empty
} else {
    # file has some content
}
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Alexander