#!/usr/bin/perl -w

use strict;

my $a = "first";

makejoin( 'second', 'third');
makejoin( undef, 'third');
makejoin('second', undef);

sub makejoin {
    my ($b, $c) = (@_);
    my $join = " where ";
    $join .= defined ($b) ?
        $b . (defined($c) ? " and $c " : '')
        : $c;
    print $a, $join, "\n";
}