If you don't want to use namespaces, but to have whichever version you load provide functions in the current namespace, you can use if . . . to get conditional loading of the libraries. Just be sure that $version is set at compile time so use knows about it.

my $version;
BEGIN { $version = shift || whatever(); }
use if 1 == $version, 'interface1';
use if 2 == $version, 'interface2';
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You can play with the logic to get a default version so long as you make sure the decisions are mutually exclusive. Even that isn't carved in stone: you can make warnings.pm ignore sub redefinitions with the lexical no warnings 'redefine';.