There is something you are not telling us.

Update: your function is fine, it is your call that is the issue as Zaxo pointed out.

use strict;
use warnings;

my $str1 = "This is string 1.";
my $str2 = "String 2 this one is.";

upper ($str1, $str2);
print "$str1 $str2";

sub upper {
  tr/a-z/A-Z/ for @_;
}
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Prints:

THIS IS STRING 1. STRING 2 THIS ONE IS.
[download]