DB<22> ($pre,@comb) = split /=|:/, "Prefix1=A,B:c,d"

  DB<23> p $pattern = "$pre=" . join ":", map { "{$_}" } @comb
Prefix1={A,B}:{c,d}
  DB<24> x <"$pattern">
0  'Prefix1=A:c'
1  'Prefix1=A:d'
2  'Prefix1=B:c'
3  'Prefix1=B:d'
  DB<25>