Don't you use warnings? 
$ perl -MPOSIX -wle'print fpathconf(*STDOUT, _PC_PIPE_BUF)'
Argument "*main::STDOUT" isn't numeric in subroutine entry at -e line 
+1.
4096

$ perl -MPOSIX -wle'print fpathconf(*FOO, _PC_PIPE_BUF)'
Name "main::FOO" used only once: possible typo at -e line 1.
Argument "*main::FOO" isn't numeric in subroutine entry at -e line 1.
4096

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And yes, they are being interpreted as zero. 

$ perl -wle'print 0+*STDOUT'
Argument "*main::STDOUT" isn't numeric in addition (+) at -e line 1.
0

$ perl -wle'print 0+*FOO'
Name "main::FOO" used only once: possible typo at -e line 1.
Argument "*main::FOO" isn't numeric in addition (+) at -e line 1.
0

[download]

In reply to Re^3: Why does fpathconf give "bad file descriptor"? by ikegami
in thread Why does fpathconf give "bad file descriptor"? by madscientist

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