A fun way to accomplish this is with DynScalar from our very own japhy.

use DynScalar;
my $filter        = "user (.*) has (logged on)";
my $output_string = dynamic { "user $1 has accessed the system and is 
+$2\n" };
my $log_string    = "user bob has logged on";

if ($log_string =~ /$filter/) {
    print $output_string;
}
[download]

I don't know about using this in production code, where someone else has to maintain it. But it's a lot of fun, especially since I was just looking at this module the other day.. And really this is mostly syntactic sugar for chromatic's suggestion, which uses the same concept.

blokhead