Thanks for your feedback, it's very interesting and appreciated.
The script will run as server and therefore I'd like it to tun in tainted mode, so I figured out another way to do things:

#!/usr/bin/perl -w -T

use strict;

# builtin triggers
my %triggers;

# add triggers from params
my $trigger_name = shift @ARGV;
my $trigger_action_str = shift @ARGV;

$triggers{$trigger_name} = $trigger_action_str;

my $buff = "You are fired!";

foreach my $trigger ( keys ( %triggers ) )
{
  if ( $buff =~ /$trigger/ )
  {
    my $c1 = $1;
    
    my $txt = $triggers{$trigger};
    $txt =~ s/\$1/$c1/g;

    print $txt;
  }
}
[download]

Yes, that's not elegant for I should save each memory container $1, $2, and so on to be general enough, but at least I can run it in tainted mode:
perl -T triggers4.pl "You are (\w+)." "Your status: $1"

So I get:
Your status: fired

Cheers,
tceng