I try to catch into a variable this warning: 
Useless use of a variable in void context

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I have tried to trap it with $SIG{__WARN__} but it doesn't work outside of a BEGIN block, and that's what I want. Here's some code: 
lorien:~$ perl -wle '$SIG{__WARN__}=sub{print"TRAPED"};s///'
TRAPED
TRAPED
lorien:~$  perl -wle '$SIG{__WARN__}=sub{print"TRAPED"};$.,$.=0'
Useless use of a variable in void context at -e line 1.
lorien:~$  perl -wle 'BEGIN{$SIG{__WARN__}=sub{print"TRAPED"};}$.,$.=0
+'
TRAPED

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I have tried to use it inside an eval block: 
lorien:~$  perl -wle '$SIG{__WARN__}=sub{print"TRAPED"};eval q{$.,$.=0
+;}'
lorien:~$
lorien:~$  perl -wle '$SIG{__WARN__}=sub{print"TRAPED"};eval {$.,$.=0;
+}'
Useless use of a variable in void context at -e line 1.
lorien:~$

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Any idea of why that happens and how to catch it without a BEGIN block?
$anarion=\$anarion;

s==q^QBY_^=,$_^=$[x7,print

In reply to catching warnings by Anarion

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