while (<DATA> and !$flag){print;}
error is: Use of uninitialized value $_ in print at C:\testwhile.pl line 8, <DATA> line 1.

while ($_=<DATA> and !$flag){print;}
error is: Value of <HANDLE> construct can be "0"; test with defined() at C:\testwhile.pl line 6.

while (defined($_=<DATA>) and !$flag)
no error