$/ = undef;
my $input = <main::DATA>;
my $d = 0;

$_ = $input;
s/\s(\d+)\/(\d+)\/(\d{4})\s/{$d = 10000*$3+100*$1+$2 if $d < 10000*$3+100*$1+$2}/eg;

print $d;

__DATA__
File:tfo.supplement.20120901.done   9/1/2012    3:48:00 AM 
File:tfo.supplement.20120901.zip    421144 KB   9/1/2012    3:48:00 AM 
File:tfo.supplement.20120904.done   9/4/2012    3:57:00 AM 
File:tfo.supplement.20120904.zip    421794 KB   9/4/2012    3:57:00 AM 
File:tfo.supplement.20120905.done   9/5/2012    3:28:00 AM 
File:tfo.supplement.20120905.zip    421875 KB   9/5/2012    3:28:00 AM