package MyBar;
...
sub Bar {
  my $val = XV_bar(@_);
  return $val;
}
...
# do the XS imports...




# and your code that calls the bar

# Foo.pl
use strict;
use warnings;
use MyBar;

my $n = 1 + MyBar::Bar(...);
# ^ warning will be generated here, not in your module, Bar.pm.