This is not going to work as written. use statements get executed in the compilation phase. if statements are executed at runtime. Even inside a BEGIN block the code gets compiled first before being executed.

The following might work:

BEGIN {
    if (completely_boring()) {
        eval <<USE;
        use Module::A;
        use Module::B;
        use Module::Etc;
USE
    } else {
        eval "use Module::C";
    }
    die $@ if $@;
}
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Putting the use statements inside an eval postpones their evaluation till runtime. This only works if the code is in a string rathern than a block. I've also used this trick to set up END blocks conditionally.

I vaguely recall that there might be a CPAN module that would let you do this more cleanly, but don't quote me on that.