The thing you're trying to do is known as "symbolic referencing", and yes, it can be done. You don't want to do it though (there are plenty of nodes around that can tell you why). One better way of doing this would be to use a hash:

use warnings;
use strict;
use Data::Dumper;

my $message_log = "/var/log/messages";

my %messages = (
        error   => {},
        alert   => {},
        warning => {},
);

open(my $logfile, "<", $message_log)
        or die "Unable to open $message_log: $!\n";
foreach my $type (keys(%messages)) {
        seek $logfile, 0, 0;
        foreach my $line (<$logfile>) {
                $messages{$type} .= $line if($line =~ /$type/);
        }
}
close($logfile);

print Dumper(\%messages);
[download]

This probably isn't the most efficient algorithm for this type of thing, but it wouldn't surprise me if it was faster than your version using shell tools.