For fiddling with little bits of code, just use the debugger straight away:

swedish_chef> perl -demo

Loading DB routines from perl5db.pl version 1.32
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   mo
  DB<1> $string = "one two three four"

  DB<2> x $string =~ m/(\w+)/g
0  'one'
1  'two'
2  'three'
3  'four'
[download]

Note that "my" variables don't work as expected, I think they get created in the Debug scope, and not in the interpreted scope. But otherwise, have fun in the sandbox.