There is an implicit check for definedness when you do while (<FOO>) { }. See the perlop section on I/O Operators.

while (<FOO>) {
  # do stuff
}

# terminates if we get a line that evaluates to false.
while (my $l = <FOO>) {
  # do stuff
}

# only terminates at end of file (or error reading file)
while (defined(my $l = <FOO>)) {
  # do stuff
}

# Let's take advantage of $_ and be somewhat safe(r).
{ local $_;
  while (<FOO>) {
    # do stuff
  }
}
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