Here's another take on the first example that only returns one line at a time until all filehandles are exhausted and returns undef when that happens.

sub gen_fh_iterator {
  my ( $current, @fhs ) = @_;
  return sub {
    my $line;
    while ( ! defined $line && @fhs ) {
      $line = <$current>;
      $current = shift @fhs
        if not defined $line;
    }
    return $line;
  };
}

my $file_iter = gen_fh_iterator( @fhs );
while ( my $line = $file_iter->() ) {
  print "$line\n";
}
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