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{
   local $_;
...
      ...
   } while defined $_;
}

for (;;) { # Loop until "last".
   local $_ = <INHANDLE>;
   ...
   last if not defined $_;
}

{ # Loop while "redo".
   local $_ = <INHANDLE>;
   ...
   redo if defined $_;
}

my $block = sub { ... };
&$block while <INHANDLE>;
&$block foreach undef;

for (;;) { # Loop until "last".
   local $_ = <DATA>;
...

   last unless defined $_;
}

a
b
c
[undef]