The simple answer is that you need first to run 
my $stream = Math::GSL::gsl_fopen $file, $mode;

[download]
The complicated answer is that gsl_fopen is defined in Math::GSL, which uses the fopen function from Math::GSL::Vector (for some reason), which is an alias of Math::GSL::Vectorc::fopen, whose definition I can't find—so I can't tell you anything but the obvious about $file, and nothing at all about $mode, except that it can end in +b.

For future reference, it would have been perfectly OK to continue our discussion in the Chatterbox.

In reply to Re^3: How do I make a histogram Question? by JadeNB
in thread How do I make a histogram Question? by Niner710

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