$ perl test.pl            # no debugger, not utf8
ID is XYZ_ABC_123456.7890

$ perl test.pl u          # no debugger, utf8
ID is XYZ_ABC_123456.7890

$ perl -d test.pl         # debugging, not utf8

Loading DB routines from perl5db.pl version 1.28
...

main::(test.pl:6):      my $filedata = "Here is XYZ_ABC_123456.7890 Foo Bar";

DB<1> c
ID is XYZ_ABC_123456.7890
Debugged program terminated...

DB<1> q


## and now the kicker:

$ perl -d test.pl u       # debugging, utf8

Loading DB routines from perl5db.pl version 1.28
...

main::(test.pl:6):      my $filedata = "Here is XYZ_ABC_123456.7890 Foo Bar";

DB<1> c
perl(18841) malloc: *** vm_allocate(size=4294832128) failed (error code=3)
perl(18841) malloc: *** error: can't allocate region
perl(18841) malloc: *** set a breakpoint in szone_error to debug
Out of memory!
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.

DB<1> q

$ perl -v

This is perl, v5.8.6 built for darwin-thread-multi-2level
(with 2 registered patches, see perl -V for more detail)