unless (open (USERNAME, "username")) 
   {
     print "ERROR:Cannot open username file.\n";
     exit(1); # ERROR
    }
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Just a parenthetic note: The code quoted above is more idiomatically (and, IMHO, better) written as below, which uses both a lexical filehandle and the three-argument form of open. (See also perlopentut.)

my $filename = 'foo';
open my $filehandle, '<', $filename or die "opening '$filename': $!";
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