Capture what should remain, not what should change: 
$s =  "test string x  x\nFeb  1 09:12:33  (";
$s =~ s/^(\w{3})  /$1 /m;

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Also note that if you are matching multiline string, the /m modifier is needed to match after a newline.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: contextual substitution with s///? by choroba
in thread contextual substitution with s///? by Wiggins

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