eval {
    require 5.6.1;
};

warn "You do not have the current version of perl." if ($@);
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Although if you plan on exiting, you might as well just do:

use 5.6.1;
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or:

require 5.6.1;
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And let Perl do the rest of the work (it die()'s if the verion of Perl on that machine is less than the one you specify).

(BTW, I believe 5.8.x is the "current" version of Perl, not 5.6.1).