use strict;

my %handles = ();
sub open_one {
    my $pid = shift;

    return $handles{$pid} if exists $handles{$pid};

    while( (my @k = keys %handles) > $threshold ) {
        my $toclose = $k[ int rand @k ];
        close $handles{$toclose};
        delete $handles{$toclose};
    }
    open $handles{$pid}, ">>", "logfile-$pid.log" or die "hrm: $!";

    return $handles{$pid};
}

sub scanner {
     # ... boring things

     if( m/something/ ) {
         my $pid = $1;
         my $fh  = &open_one($pid);
         print $fh $stuff;                 
     }

     # boring things...
}