useperlio is an option you set when compiling/installing perl http://search.cpan.org/dist/perl/INSTALL 
$ perl -V |grep -i perlio
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=und
+ef
                        USE_LARGE_FILES USE_PERLIO USE_SITECUSTOMIZE

$ perl -V:useperlio
useperlio='define';

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In reply to Re^3: perl printf vs C fprintf performance by Anonymous Monk
in thread perl printf vs C fprintf performance by decnartne

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