You're almost there.

When you're using a regular expression, the regexp itself can't be a variable (the contents between the / delimiters can). So, for the above:

my $STR = "AAXXXAA";
$STR =~ s/AA/BB/g;
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Rather than assigning to the regular expression to a variable, then using the =~ operator on it, you need to apply the regular expression directly.

If the contents of the regexp are going to change at runtime, you can place these into variables:

my $sub_str = "AA";
my $sub_val = "BB";

my $str = "AAXXXAA";
$str =~ s/$sub_str/$sub_val/g;
[download]

Hope this helps ..
--Foxcub