See use. use Foo::Bar; is basically BEGIN { require Foo::Bar; }. If you want some conditional logic, you can put that into the BEGIN block as well:

BEGIN {
    if( $ENV{USE_XYZ}) {
        require Foo::Bar;
        Foo::Bar->import();
    }
}
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The if module wraps that simple case already:

use if $ENV{USE_XYZ}, 'Foo::Bar';
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