Is this line 16?

open (WORK, "<$work_file") or die "Couldn't open $work_file."; # Open 
+the working file
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If so, maybe it's a backslash thing (sorry... I don't do Windows). Try this simplified code:

use strict;
use warnings;

my $work_file = 'E:\triggers\1_MERGE_3326\tests\20131209_180245424_R01
+.hl7';
open (WORK, "<$work_file") or die "Couldn't open $work_file : $!";
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Tip #7 from the Basic debugging checklist ... $!