If you don't really care about leaving the JSON intact, your problem at hand seems to be that Perl regards { and [ and . as special within regular expressions.
To temporarily switch off the special treatment, see the \Q and \E escapes in perlre. Basically, your regexp could be written as:
s/\Q{"absoluteLimits":{"conditionalLimits":[{"bidirVolume":102400000,"
+name":"Home"}],"resetPeriod":{"volume":"2400 hours"}},"description":"
+Promotion_tariffbasic:\E.*//s
(find where I added \Q and \E in the Perl code)
