my $string = "[";
print "\Q$string";
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perldoc -f index

index STR,SUBSTR,POSITION
index STR,SUBSTR
      The index function searches for one string within another, but
      without the wildcard-like behavior of a full regular-expression
      pattern match. It returns the position of the first occurrence
      of SUBSTR in STR at or after POSITION. If POSITION is omitted,
      starts searching from the beginning of the string. The return
      value is based at "0" (or whatever you've set the "$[" variable
      to--but don't do that). If the substring is not found, returns
      one less than the base, ordinarily "-1".
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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).

You don't need to do all this, it's what \Q is for!

Perhaps I should have been more explicit...

my $string = 'abcdef';
my $match = '[a-z]';

$string =~ s/\Q$match/NOT REPLACED/;
print "$string\n";
$string =~ s/$match/REPLACED/;
print "$string\n";

__OUTPUT__
abcdef
REPLACEDbcdef
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have a look in ...

perldoc perlre
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---
my name's not Keith, and I'm not reasonable.