Your code works on my machine when the variables $sanity_code and $tests are simple short strings.

I suspect that the problem is that further up in the code wherever those variables are calculated, there is some humongous result. Try just printing $sanity_code and/or $tests to STDOUT instead of to PACK and see what happens....

#!/usr/bin/perl
use strict;
use warnings;

my $pack = 'test.txt';     #fine when using these vars....
my $sanity_code = "blah";
my $tests = "asdfasdf";


   open(PACK,">$pack") or die("failed to open file $pack");
   print PACK <<_M_E; 
#!/usr/intel/bin/tcsh -f
$sanity_code
$tests
_M_E

   close(PACK) or die("failed to close file $pack");
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