If you call via perl backticks then only "hello" is captured, and "world" is still sent to stdout.
I cannot reproduce that. 
$ cat foo
#!/usr/bin/perl
print STDOUT "hello\n";
print STDERR "world\n";
$ cat bar
#!/usr/bin/perl
use 5.010;
my $output = `./foo 2>&1`;
say "[[$output]]";
$ chmod +x foo
$ chmod +x bar
$ ./bar
[[world
hello
]]
$

[download]
Or are you suggesting the OP would run a different command in backticks than in his open? Why would he do that?

In reply to Re^3: capturing command output by JavaFan
in thread capturing command output by morgon

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