You're right, and i seem to have overseen the fact that use; statements take execution precedence. Moreover, this particular eval() doesn't work too well as a truth assertion (blaming use) and the subsequent use; gets executed no matter what.

Why fallback to the same module?? Well, maybe we do need it but ideally we'd like to take control and load the one version of it identified by a supplied version number but if the module at that version isn't available, prepare the script for the standard version and continue, something along the lines of OP's wants.

So, as has already been examined, "use lib '../'; use Foo::Bar;" seems to have worked simply and elegantly.. but "use lib" places entries at the beginning of @INC and what if non-standard versions of ../strict.pm or ../warnings.pm existed? The user might inadvertently load these modules (OUCH!!) rather than the ones in the original @INC. Placing entries at the beginning of @INC is a bad idea, the risk of polluting the package with bogus code as a result of namespace clashes is real. 
{
    use lib "..";
    use strict;  # "../strict.pm" loaded instead of the real one
}

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Placing entries at the end of @INC means that modules if found in the original @INC take precendence and are loaded instead of the ones you want loading. The chances are of such a case are slim but entirely possible especially if the user isn't aware of the perl module namespace. Even otherwise, the user might be patching/updating the an already existant module and can ensure the testing release is loaded by using it's $VERSION (wherever the new .pm is located via @INC). Placing entries at the end of @INC is generally safer.

As an added benefit, using the $Foo::Bar::VERSION to validate loading Foo::Bar also ensures that your code is guaranteed to run under Foo::Bar, alternate versions of the module might break something.

So if the OP wanted to load a non-standard Foo::Bar (let's assume v5.00) instead of a standard Foo::Bar (v5.01) and provided the version numbers reported by the 2 modules were different (they really ought to be) ... 
#!/usr/bin/perl -W

BEGIN { push @INC, ".." }

use strict;              # "../strict.pm" untouched

eval "use Foo::Bar 5.01 qw| raz baz taz |";  # try loading v5.01
if ($@) {                # if loading v5.01 failed
  warn "@_";             # fallback to using standard (v5.00)
  use Foo::Bar qw| baz |; # raz and taz arent valid tags here
}
...

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In reply to Re^3: BEGIN and compile-time by Firefly258
in thread BEGIN and compile-time by jbert

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