$ perl -Mre=debug -e " m//"
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Matching REx "" against ""
   0 <> <>                   |  1:NOTHING(2)
   0 <> <>                   |  2:END(0)
Match successful!
Freeing REx: ""

$ perl -Mre=debug -e " $_ = q/fi/; m//"
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Matching REx "" against "fi"
   0 <> <fi>                 |  1:NOTHING(2)
   0 <> <fi>                 |  2:END(0)
Match successful!
Freeing REx: ""