I'll go back and make the changes you suggested. I thought I used the exact syntax for Printer stated in the documentation.

Here's the kicker, it's not always failing. Even though I'm explicitly setting ENV{PATH} in all 3 files, every 3 or so times the script runs, it somehow resets the ENV{PATH} to the original value. Only when I explicitly reset it right before it fails in Printer::Unix.pm does it get the correct ENV{PATH}.

$ENV{PATH}='/usr/bin';
warn "path=$ENV{PATH}\n";
open PRINTER, "| $self->{print_command}->{linux}-{command}"
    or Carp::croak..
[download]

...And then it will again randomly fail (more like every 6th running instead of every 3rd), with the new error Can't open printer connection to lp -d 6L.

ETA: However, if my code is:

warn "path=$ENV{PATH}\n";
open PRINTER, "| $self->{print_command}->{linux}-{command}"
    or Carp::croak..
[download]

(without the redundant 4th reset of ENV{PATH}), sometimes the warning output shows the correct PATH, sometimes, it doesn't.