No need to declare that variable; just use the arrays. Maybe you're looking for this (untested):

use warnings;
use strict;
use diagnostics;

use File::Copy;

# Set up arrays with original name and new name with unique ISBN
my @IDI_name = ('I:\Production\TEST\P001.txt', 'I:\Production\TEST\P00
+2.txt'); 
my @PLL_name = ('I:\Production\TEST\ISBN_Text.txt', 'I:\Production\TES
+T\ISBN2_Text.txt'); 

# Walk through array and copy with new filename
for my $i (0 .. $#IDI_name) {
    copy($IDI_name[$i], $PLL_name[$i]);
}
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