Finally, I tried the following code, it works perfectly for monitoring all the files in a directory! Thanks you all!

use File::Monitor;
use File::Copy::Recursive qw(fcopy rcopy dircopy fmove rmove dirmove);

my $monitor = File::Monitor->new();

$files= 'c:\\PAPERS';
     opendir(AA,$files) || return;
     my @list = readdir(AA);
     closedir (AA);
foreach $file (@list) { $monitor->watch( $file ); }

$monitor->callback( mtime => sub {
    my ($file, $event, $change) = @_;
         print "$file has been modified \n\n";
         fcopy ("$file","p:\\phdpaper");
    } );
    
while (1) {
    $monitor->scan;      
    sleep 10;
    # After the first scan we get a list of  File::Monitor::Delta obje
+cts
    # that describe any changes
    my @changes = $monitor->scan;
    for my $change (@changes) {
        # Call methods on File::Monitor::Delta to discover what change
+d
        if ($change->is_mtime) {
            my $name = $file_name;
        }
    }
    
 }
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