...because you typed 'use ThisModule;' at the top of your code is the simple answer -- since use'ing occurs at compile-time, then the statement must already occur in your code. You cannot load a module twice, if it has already been loaded with use or require. This also means that you can't have two different versions of the same module loaded at one time either.

if you are talking dynamic inclusion of module at runtime, or in response to user responses etc, then the perl idiom you are looking for is:

if ( $i_need_a_dynamically_loaded_module )
{
    eval "require Module";
    $@ && warn $@;
}
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if you have a list of modules which could do the job but you don't know what the host has, then either traverse @INC manually (clumsy), or try something like:

my $loaded_mod;
foreach my $mod ( qw( Module1 Module2 Module3 ) )
{
    warn "attempting to load '$mod'";
    eval "require $mod";
    next if $@;

    warn "'$mod' loaded successfully";
    $loaded_mod = $mod;
    last;
}
$loaded_mod or die "Couldn't load anything useful...";

...
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dirty