#!/usr/bin/perl
use DBI;
$|=1;
my $dbh = DBI->connect('DBI:mysql:foodb:localhost', 
                       'user', 'pass');
my $sth = $dbh->prepare("SELECT a FROM table
                        WHERE b = ?");
my $b;

    while (my $a = <STDIN>) {
        chomp $a;
        $sth->execute($a);
        $sth->bind_columns(\$b);
        $sth->fetch;

        print "$b\n";
    }