The files are not in the current directory. You need to supply the path to stat as well.

Probably something like:

use File::stat;

my $path = '/to/my/directory';

opendir(DIR, $path) or die("Cannot open directory"); 
my @files= readdir(DIR); 
closedir DIR;

foreach my $file (@files) {
my $mod = (stat("$path/$file"))[9];
print "Filename $file has the mod date of $mod \n";
}
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