The only way you are going to get around the syntax problems is to only compile the code in question if the module exists....

eval "use Foo::Bar";

eval <<"END" unless $@;
sub my_method { # Code }
END
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but because that is going to introduce it's own problems, I would opt for moving that code into a module or a library...

eval "use Foo::Bar";
if ( $@ ) {
    require "have-foo-bar-library.pl";
} else {
    require "dont-have-foo-bar-library.pl";
}
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As for the second part, because you are using eval BLOCK rather than eval STRING, the use will still be evaluated at compile time, rather than at run time. You could get the effect you are looking for by breaking the use up into it's components though...

eval { require Foo::Bar; import Foo::Bar; }
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