I am unable to reproduce your findings. After removing the apparently unnecessary use utf8::all; the script runs fine to completion with no warnings or errors:

$ ./1178644.pl "$FOO"
$ perl -v

This is perl 5, version 20, subversion 3 (v5.20.3) built for x86_64-li
+nux-thread-multi
(with 16 registered patches, see perl -V for more detail)

Copyright 1987-2015, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

$

[download]

with $FOO set to be "tekst=Οὐχὶ ταὐτὰ παρίσταταί". This is using version 4.21 of CGI. Which version are you using and what input are you supplying to produce the stated error message?

In reply to Re: Can't decode ill-formed UTF-8 octet sequence <FF> at /usr/share/perl5/CGI.pm line 1116. by hippo
in thread Can't decode ill-formed UTF-8 octet sequence <FF> at /usr/share/perl5/CGI.pm line 1116. by nikolay

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