in reply to Re^2: Regex triggering uninitialised values
in thread Regex triggering uninitialised values

Then you'd have 
$str =~ s/(\d*)%var%(.*)/$1REPLACED$2/;

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Although, unless you're going to use $1 and/or $2 later, there's no reason to have them at all. 
$str =~ s/%var%/REPLACED/;

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