use:

Specifying VERSION as a literal of the form v5.6.1 should generally be avoided, because it leads to misleading error messages under earlier versions of Perl (that is, prior to 5.6.0) that do not support this syntax. The equivalent numeric version should be used instead.

use v5.6.1;     # compile time version check
use 5.6.1;      # ditto
use 5.006_001;  # ditto; preferred for backwards compatibility
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