stddev() is not a built-in function, it is a function provided by the module. From looking at the docs that module does not export any functions by default so you will need to import them before using them: 
# you can remove this line
use Statistics::Basic::StdDev;

# add this line
use Statistics::Basic qw(:all);

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You can then use all the functions in your script.

In reply to Re: Annoying error although module is installed by tangent
in thread Annoying error although module is installed by Anonymous Monk

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