next unless $description =~ /$regexp/i;
        $severity = $2;
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If we reach this second line, the match must have succeeded, which would have reset $2 to undef.

A simpler way to do this is to use a hash for %severity_map. Heck, you already call it a map, but it's not in a normal form -- you have to unpack/decode it (with split) every time you use it to get to the actual data. That's a sure sign that you should rethink a data structure. With a hash, you can avoid having nested loops, and simply perform one regex match:

my %severity_map = (
     "Error"                    => "Warning",
     "Critical"                 => "Critical",
     "System Shutdown"          => "Fatal",
     "Failure"                  => "Critical",
     "Uncorrectable ECC"        => "Fatal",
     "Warning"                  => "Minor",
     "Alarm"                    => "Critical",
     "System Powered Off"       => "Fatal",
     "Memory Bank Deconfigured" => "Warning"
);

my @description_samples = (
    "Memory Bank Deconfigured",
    "Uncorrectable ECC",
    "Sticky Corrected ECC Error",
    "System Shutdown"
);

my $keywords = join "|" => map quotemeta, keys %severity_map; 

for (@description_samples) {
    my $severity = "Warning";

    /($keywords)/i
        ? $severity = $severity_map{$1}
        : print "Can't find severity mapping for: $_, using default\n"
+;

    print "$_ has severity $severity\n";
}
[download]