use utf8; does not use result in any code. 

>perl -MO=Concise,-exec -e"use utf8; $x='abc';"
1  <0> enter
2  <;> nextstate(main 7 -e:1) v:U,{
3  <$> const[PV "abc"] s
4  <#> gvsv[*x] s
5  <2> sassign vKS/2
6  <@> leave[1 ref] vKP/REFC
-e syntax OK

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It has a compile-time effect.

In reply to Re^3: avoid writing 'use utf8' in every script by ikegami
in thread avoid writing 'use utf8' in every script by reqnode

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