Any reason why you can't pass the value into libcall() directly rather than via a localised %ENV element? (Which seems a strange way to do things?)

sub libcall {
    my $arg = shift;
    if( $arg ) {
        print "Arg is $arg\n";
    } else {
        print "Arg undefined\n";
    }
}


libcall( condition() ? 42 : undef );
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