require Module1;
require Module2;

Module1::foo();
Module2::foo();

##</code><code>##

my $mod = some_condition() ? 'Module1' : 'Module2';
eval "require $mod; ${mod}::foo()";

##</code><code>##

use strict;
use warnings;

my $mod = shift;

require "$mod.pm";

eval "$mod->import(); 1" or die;  # modules all export 'foo' by default
# eval "$mod->import( 'foo' ); 1" or die;

foo();