my $function = sub {
...
};

if($isDebug) {
    $function = sub {
        ...
    }
}

##</code><code>##

my $getDiceRoll = sub {
    return int(rand() * 6) + 1;
};

my $dicefh;
if($regressionTest) {
    $getDiceRoll = sub {
        # READ NEXT DICE ROLL FROM dicerolls.txt
        if(!defined($dicefh)) {
            open($dicefh, '<', 'dicerolls.txt') dir die($!);
        }
        my $roll = <$dicefh>;
        chomp $roll;
        return $roll;
    }
}