Functions do not assign to $_ by default. Consider

sub foo { 'foo' };
foo();
print $_,"\n";
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You wouldn't expect that to print 'foo', would you? Well, the while loop doesn't make it any different. You have probably been confused by things like while (<>) {} (which works because <> assigns to $_ by default) and for (@list) {} which works because for will use $_ by default when an iteration variable isn't named.

-sauoq
"My two cents aren't worth a dime.";