Actually, perldoc perlop, in the section on "I/O operators", is very explicit about it:

Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;)loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously.
(emphasis mine)

Do not rebuke them with harsh words ... but rather lead them gently - with URLs - so that they may learn wisdom.

In reply to Re^2: Uninitialized string warning by VSarkiss
in thread Uninitialized string warning by RICH15ARD

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