Blakem pointed you to a good resource, but basically you've hit Context. In Perl, when you:

Do a mathsy thing to a string (like '0E0' or 'oink' or '57Fred')

Any digits at the start are taken as the value and the rest is lost. So (5 + '0E0') == (5 + '0') == (5 + 0) == 5

Do a stringy thing to a string

It stays the way it is

Do a neutral thing to a string, like checking for truth

It again gets left the way it is.

So in your case, '0E0' is true as a string, because it's neither empty nor just the string '0', but false as a number because it gets whittled down to the value 0 (the digits at the start).