Just a few random comments from another set of eyes:

#!/usr/bin/perl
# This script watches the /var/log/secure
[download]

So it runs as root, but you're not using taint mode. No bugs at first sight, but you never know.

my @safe = qw/ 192.168.0.1 /;
[Some code]
my @rejects;
[download]

Why not define @rejects just under my @safe since it serves a similar purpose?

next unless $line =~ /Failed password/ig;

If I were you, I'd tight that regex a little more to avoid false positives, althought that has the drawback that maybe in a future version of the sshd daemon, the message about failed login attempts changes and suddenly you don't seem to be attacked anymore ;^). Also, why /ig?

if ( !defined( $db{$ip} ) ) {
    push ( @{ $db{$ip} }, $epoch );
}
else {
    push ( @{ $db{$ip} }, $epoch );
[download]

The push could be moved outside the if. Plus, is there more information stored in $db{$ip}? You seem to store only the IP, so there's no need to have a hash and use push and undef below. Just $db{$ip} = $epoch.

my @safe = @_;
foreach (@safe) {
    return 1 if $ip eq $_;
}
return 0;
[download]

If you like compact code, try: return !!grep { $ip eq $_ } @_ ## tested

sub isRejected {

Either rename this to rejected, or safe to isSafe.

foreach (@rejects) {

Same grep as before.

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