my ($buffer, $command);
while(read(STDIN, $buffer, 1)){
    #print "Read Character $buffer\n";
    if($buffer eq "\t"){
        ##Do command completion stuff
        last;
    }else{
        $command .= $buffer;
    }
}

if($command eq "this"){
  #Stuff
}elsif($command eq "this"){
  #More Stuff
}...