Problem with uninitialised value errors when using perlfunc '-M' option to obtain file timestamps

Windows OS environment

#filets.pl
use strict;
use warnings;
use diagnostics;

my $dir = 'subtest\\';
opendir(DIR, $dir) or die $!;
while (my $file = readdir(DIR))
{
    print "$file has -M timestamp of:", -M $file, "\n";
}
closedir(DIR)
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***Sample Output***

. has -M timestamp of:0.0101967592592593

.. has -M timestamp of:6.89083333333333

Use of uninitialized value in print at filets.pl line 13 (#1)...

sub2.txt has -M timestamp of:

SubSample.txt has -M timestamp of:

***End of Sample Output***

Notes: the '-M' option works for me without problems for reading files located in the same directory where the perl script is located

I just can't seem to get it to work when reading files in another directory

Any help would be appreciated on how I can get '-M' option to work for files in a subdirectory