I'm sure there are better ways, but the following seems to work:

#!/usr/bin/perl -w
use strict;
use Getopt::Long;

my $line;
my $NUM;
my $FILE;

GetOptions("f=s" => \$NUM,    # -f=/var/log/messages
           "n=s" => \$FILE);  # -n=30

if (!($NUM)) {
        $NUM = 20;
}

my @TailFile = `tail -$NUM $FILE`;

foreach $line (@TailFile) {
        chomp $line;
        print "$line\n";
}
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Update: My comments were flip-flopped. Corrected.

If things get any worse, I'll have to ask you to stop helping me.