Correct me if I'm wrong, but OP said:

Turns out it was because the PATH didn't exist in the Jenkins environment.
Why would the open succeed to an unknown executable?

As far as I understood OP, missing PATH meant that the script was unable to run executables except by full path:

$ env LC_ALL=C PATH="" /usr/bin/perl -E'open my $fh, "-|", $_ and say 
+"success" or say $! for "ls", "/bin/ls"'
No such file or directory
success
[download]

So the question was: how does open succeed despite inability to find the executable due to empty $PATH?

_{By the way, thanks for the spelling reminder.}