my $file1 = 'c:\test1.xls';
my $file2 = 'c:\sample.zip';

my $file3 = $file2 || $file1; # file to attach
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As you're using logical 'or', and $file2 is true (non-empty), $file3 will be $file2. And $file3 is the only file you're ever opening/reading...