Mongoid：检索所有嵌入的文档(Mongoid: Retrieves all embedded documents)

 你有什么应该工作， 但是 ：不要使用qx 。 这是古老而危险的; 无论你向它提供什么都通过shell，所以如果/bin/sh不是你/bin/sh那么很容易受到shell注入或者遇到意外。 
 
 使用open()的多参数形式，它完全绕过了shell。 
 
open my $fh, '-|', 'df', "-$opt", '/tmp' or die "Can't open pipe: $!";
my @lines = <$fh>;  # or read in a loop, which is more likely what you want
close $fh or die "Can't close pipe: $!";

What you have should work, but: don't ever use qx. It's ancient and dangerous; whatever you feed to it goes through the shell, so it's very easy to be vulnerable to shell injection or run into surprises if /bin/sh isn't quite what you expected.
 
Use the multi-arg form of open(), which bypasses the shell entirely.
 
open my $fh, '-|', 'df', "-$opt", '/tmp' or die "Can't open pipe: $!";
my @lines = <$fh>;  # or read in a loop, which is more likely what you want
close $fh or die "Can't close pipe: $!";