FWIW, I made a smaller reproducer:
$ create vms_problem.t
use strict;
use FileHandle;
use Encode (":all");
{
no warnings;
}
open my $fh, '<:encoding(euc-kr)', 'ksc5601.enc' or die "ksc5601.enc : $!";
my $dtext = join('' => <$fh>);
close $fh;
^Z
$ perl vms_problem.t
euc-kr "\xAD" does not map to Unicode at vms_problem.t line 9, <$fh> line 230.
I guess the next thing is to create a build for the VMS debugger and go
slogging around in the vicinity of PerlIO_read, though I'm not sure exactly
what I'm looking for.