After a quick glance over the documentation to \Q...\E I'd try

$cache_ref->{Index}[$i]=~/(\Q$fromto_digest\E)/ or ref->{Index}[$i]=~/
+(\Q$tofrom_digest\E)/)
[download]

because I read the docu as to mean, that in your case the parens and everything will be quoted after the \Q

I'm tired, so don't count on me ;-)

regards,
tomte

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Edit: ++pfaut, I knew there was something 'fishy' but couldn't get it