The following code does not print any warning for me:

>perl -wle "my $date='20080501'; $date =~ s/^(\d{4})(\d{2})(\d{2})$/$1
+-$2-$3/;print $date"
2008-05-01
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So the problem must be with your other code and/or how you're using it. My guess is that $date in your case is not defined. Print out $date before trying a substitution on it:

print "I think \$date is [$date]\n";
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