Let Perl help with your debugging. Enable warnings and strictures.

use strict;
use warnings; # or add the -w flag on your shebang line

my $sql = "...";
my $dbh = ...;
my $sth = ...;

while (<>) {
    chomp;
    next unless $_;
    my ($sn, $ii) = split(/\s+/, $_, 2);
    $sth->execute($sn, $ii) or warn $DBI::errstr;
    if (my ($s, $i) = $sth->fetchrow_array()) {
        print "Serial Number: $s Item ID: $i\n";
    } else {
        print "Select failed for ($sn) <$ii>\n";
    }
}
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