You just need to escape the characters with a \. Of course, that's also a special character...

my $match = '(some [very] odd value).';
my $replace = '(modified value)';

# escape anything that might be a potential problem.
# note -- we ignore known good values, rather than
# encoding known bad values (so if we don't know about,
# it gets encoded)
$match =~ s/([^\w\s])/\\\1/g;
my $regex = qr/$match/;
my $SomeStuff = 'This is a test string (value); (some [very] odd value
+).';
$SomeStuff =~ s/$regex/$replace/g;

print $SomeStuff;
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Update: Sorry, I didn't know about \Q ... take a look at reasonablekeith's response. (the logic from mine might be useful if you're trying to force some alternate escape sequence, if there's a possibility of wanting to escape some characters, but allow some to pass through untouched).