The operating system is telling your Perl script what the problem is; your Perl script is just not reporting it. Don't ignore the operating system when doing operating system operations such as copying files.

use strict;
use warnings;

use English qw( -no_match_vars );
use File::Copy;

my $copy_from = 'C:/Users/Admin/Desktop/Staples Files/ReadFile.txt';
my $copy_to   = 'C:/Users/Admin/Desktop/Staples/Staples_ReadFile.txt';

copy($copy_from, $copy_to)
    or die "Can't copy file from $copy_from to $copy_to:\n$OS_ERROR\n"
+;

exit 0;
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