Two things:

1. If you know you want to output xml, then print header('text/xml'); is a lot simpler.
2. Always check to see if your 'open' succeeded. Your error message means that your script didn't open the file.

#!/usr/local/bin/perl
use strict;
use warnings;
use CGI qw(:standard);
use CGI::Carp qw(fatalsToBrowser);

print header('text/xml');
my $file = "/some/file.xml";
open(XMLFILE, $file) or die "Failed to open $file, $!";
while (<XMLFILE>){
   print;
}
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