For some reason the import() routine is not being run, and so the function is not injected into your namespace.

As a simple matter of code hygiene, you could explicitly state what you want exported from File::Basename: use File::Basename 'basename';

And who knows, it might fix your problem. If it still fails to work, does the following work?

my $base = File::Basename::basename($gds_file,".soft");
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If that still fails to work, does perldoc -m File::Basename show you something that resembles the code of the module itself?