I'm not sure if I'm missing some caveats, but I usually attack problems like this by pushing values into the @INC array in a BEGIN block. The code below assumes that the script name (or any directory names) won't contain slashes (/).

#!/usr/bin/perl -w
BEGIN {
  $0=~s#/?[^/]*$##;
  $0 ||= '.';
  push @INC,$0.'/../perl_modules';
}
print "$_\n" for @INC;
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