You should probably use one of the existing date modules, but if you want to do it yourself, take a look at a possible simple solution in this session under the perl debugger:

  DB<8> $c = time

  DB<9> p $c
1375466025
  DB<10> $c -= 3600*24

  DB<11> $d = localtime $c

  DB<12> p $d
Thu Aug  1 19:53:45 2013
[download]

which is exactly one day ago in my local time zone.