What you want is:

use warnings;
use strict;

my $start = '<?xml version="1.0" encoding="utf-8"?>
<MultifamilyProperty xmlns:xsi="http://www.w3.org/2001/XMLSchema-insta
+nce" xmlns:xsd="http://www.w3.org/2001/XMLSchema">';
my @keys = 1 .. 5;
my $output_file = ".xml";

for my $val ( @keys )
{
    $output_file = $val . $output_file;
    open my $file_handle, '>:utf8', $output_file or die "Cannot open '
+$output_file' because: $!";
    print $file_handle "$start\n\t\t<Address>\n";
}
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