my %convert = ( "Jan" => "01", "Feb" => "02", "Mar" => "03", "Jun" => "06" ,); # ... you get the idea

my $in = "Fri Jun 16 18:04:03 2012";
$in =~ /.*?\s(\w\w\w)\s(.*)$/;
my $out = $convert{$1} . " " . $2;