Another approach would be to join the lines of the result and match against that. For example:

#!/usr/bin/perl
use strict;
use warnings;

# Slurp!
my @results = <DATA>;
my $results_string = join '', @results;

# Note: Direct assignment to hash works due
# to exactly two matches. Error checking would
# not be a bad idea, either (what happens if the
# match fails?)
my %matches = $results_string =~ /(hdisk\d+).+?Serial Number\.+(\w+)/g
+s;

for my $device (keys %matches) {
    print "$device  $matches{$device}\n";
}
__DATA__
hdisk0   P1/Z1-Aa     16 Bit LVD SCSI Disk Drive (18200 MB)

        Manufacturer................IBM
        Machine Type and Model......ST318305LC
        FRU Number..................09P4437
        Serial Number...............0009E05B
        Part Number.................09P4436

hdisk1  P1/Z1-Ab     16 Bit LVD SCSI Disk Drive (18200 MB)

        Manufacturer................IBM
        Machine Type and Model......ST318305LC
        FRU Number..................09P4437
        Serial Number...............0004A0D2
        Part Number.................09P4436
[download]

Output:

hdisk1  0004A0D2
hdisk0  0009E05B
[download]