Jenkins – the Jenkins variable containing the Perforce changelist that triggered the build

To output the local path of all pending adds of a changelist you can use:

p4 opened -c changelist | grep -w add | sed 's/#.*//' \
| p4 -x - where | awk '/^\// {print $3}'

This does the same without grep but is a bit more obscure:

p4 opened -c changelist | sed -n 's/\(.*\)#.*- add .*/\1/p' \
| p4 -x - where | awk '/^\// {print $3}'

One of least cumbersome and cheap ways to do this: you could pass the A ${BUILD_NUMBER} environment variable to B by using Parametrized Build plugin. For example, define PARENT_BUILD_NUMBER variable in B and the pass PARENT_BUILD_NUMBER=$BUILD_NUMBER from A to B.