This is explained in perlop in relation to <>: 
   
    Ordinarily you must assign the returned value to a vari­
    able, 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' state­
    ment (even if disguised as a `for(;;)' loop), the value is
    automatically assigned to the global variable $_.

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--
John.

In reply to Re: When does while set $_ ? by jmcnamara
in thread When does while set $_ ? by crenz

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