You can use the VERSION method.

package Module;

use Data::Dumper;

sub VERSION
{
    my $class = shift;
    my $version = shift;

    require(sprintf("%s_%d.pm", $class, $version));

    push @ISA, $class . '_' . $version;

    1;
}

1;
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If you want to use 2.1 you will need to convert the numbers into something like 2_1, as Module_2.1 is not a valid Module name.