You're using parens where you should be using curlies. (And '_' doesn't need escaping.) 
s/\w{6}_(\d{4})/$1/;

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which is pretty much the same as the following 
s/\w{6}_//;

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The following should be faster, though: 
$_ = substr($_, -4);

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In reply to Re: Fetch numbers only by ikegami
in thread Fetch numbers only by Anonymous Monk

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