Does the script look ok to you?

A couple of things I would change is to add use CGI::Carp qw/fatalsToBrowser/; and then remove the chdir and put in the path $ENV{DOCUMENT_ROOT}/ into your $a and $b vars. I would also put the $a and $b between quotes.

#!/usr/bin/perl

print "Content-type: text/html\n\n";

use CGI;
$q = new CGI;

$a = "$ENV{DOCUMENT_ROOT}/file1.htm";
$b = "$ENV{DOCUMENT_ROOT}/file2.htm";
        
rename("$a", "$b") || die "Can't rename $!";
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