Well for starters "0" and an empty string("") are false. Anything else is basically true.

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 warni
+ng "use of uninitialized value" if you are using -w
2-3+1     # computes to 0 which is converted to "0" so it is false
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