in reply to Re: using variables in regular expr replace
in thread using variables in regular expr replace
And just to elaborate on why you need to do this, remember that  indicates a character class inside a regular expression (i.e. [a-c] means any of the characters a, b, or c). Your [img ... 8-4 ... ] is being interpreted as a character class, and you've got an invalid range (the character 8 is "bigger" than 4 and hence the correct way to specify the range of characters would be [4-8]).
At any rate you really should either escape the square brackets or use \Q...\E as shown above (and you should read perldoc perlretut and perldoc perlre).