my ($first) = /^\s*(\w+)/ or next;
if (index $first, "hello") {
    ... whatever ...
}

##</code><code>##

my %prefixes = calc_all_unique_prefixes;
if (/my ($first) = /^\s*(\w+)/) {
    if (exists $prefixes{$first}) {
        do_command $prefixes{$first}
    }
    else {
        print "What?\n";
    }
}