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).

In reply to Re: OEO, a zero that evaluates to true? by n3dst4
in thread OEO, a zero that evaluates to true? by coolmichael

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