It may be that what you want is someting like:

use strict;
use warnings;
use String::Compare qw();
use File::Compare qw();

my $str1 = "fred.txt";
my $str2 = "fred1.txt";

String::Compare::compare ($str1, $str2);
File::Compare::compare ($str1, $str2);
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but since you don't describe the error it's a bit hard to tell.