If it is being given a value by a for loop, a while loop with a readline operator, given, grep, or map, then it is defined.
Not if the value given isn't defined. 
$ perl -wE 'say for 1, undef, 3'
1
Use of uninitialized value in say at -e line 1.

3

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In reply to Re^2: uninitialized $_ sneaks past warnings in grep/map block or expression by JavaFan
in thread uninitialized $_ sneaks past warnings in grep/map block or expression by rir

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