0         # converts to "0" so it's false;
""        # is empty string so it's false;
"0.0"     # not "0" or "" so it's true
0.0       # computes to 0 and is then converted to "0" so false
undef     # evaluates to "" so it's false -- note, you may get a warning "use of uninitialized value" if you are using -w
2-3+1     # computes to 0 which is converted to "0" so it is false