Thank you for your suggestions. Something about $searching_string

my $searching_string=Encode::decode_utf8( $searching_string_widget );#
+$searching_string_widget is variable in Entry widget (tk)
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Output from

use Devel::Peek;
Dump($searching_string);
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SV = NULL(0x0) at 0x3b51f8c
REFCNT =
FLAGS = ()
PV = 0

Yes, I confirm that LOWER works with non ASCII characters. Strangly enought, the above script produces what I want (lower/upper case matches) even without LOWER (non ASCII). It just doesn't work with non western charachters (UTF8).

I did'n load the ICU extension. I must admit I have no idea how it goes. I'll try to get some infos about it.