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my $fh = IO::File->new($filename, 'r');

my $fh_iterator = sub {
...
while (my $line = $fh_iterator->($fh)) {
    # do stuff to $line
}

sub create_iterator{
   my $filename = shift;
...
while (my $line = $fh_iterator->()) {
    # do stuff to $line
}