package MyClass;
use Moose

use Try::Tiny;

use namespace::autoclean;

BEGIN {
    if ( try { require Foo; 1 } ) {
        *bar = sub {
            my $self = shift;
            Foo::foo($self->baz);
        };
    } else {
        *bar = sub {
            ... # fallback implementation
        };
    }
}