Use use strict; use warnings;. You are hiding valuable error messages if you don't. Specifically,

"my" variable $M masks earlier declaration in same scope at a.pl line 
+8.
Global symbol "$dt" requires explicit package name at a.pl line 9.
Global symbol "$dt" requires explicit package name at a.pl line 10.
Global symbol "$dt" requires explicit package name at a.pl line 12.
Execution of a.pl aborted due to compilation errors.
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Fix:

sub fromGMT { my ($date, $hms) = @_;
   my ($Y, $m, $d) = unpack('A4A2A2',$date);  
   my ($H, $M, $S) = $hms =~ m/^(\d+):(\d+)(?::(\d+))?/; $S ||= 0;  
   my $dt = DateTime->new(
      year=>$Y, month=>$m,  day=>$d,
      hour=>$H, minute=>$M, second=>$S,
      time_zone=>'GMT'
   );
   $dt->set_time_zone('America/Chicago'); 
   return $dt->hms;
}
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