Yet another slightly different way to accomplish this. Note the BEGIN is redundant

use constant ({
    START           => 1000,
    NO_OF_CALLS     => 1,
    DEBUG           => 1,
    DET_DEBUG       => 1,
    TOTAL_TIME      => 1,
    DETAILED_TIME   => 1,
    RESPONSE        => 1,
    SLEEP           => undef,
    MICRO_SEC       => 1,
});

use if MICRO_SEC, qw(Time::HiRes time);
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And before learning about the if package, I'd do the following:

BEGIN {
    if (MICRO_SEC) {
        require Time::HiRes;
        Time::HiRes->import('time');
    }
}
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