That's presumably because you left out the command line parameter '-p .'. Then it checks per default for the string "pdf".

If I just run the script without piping it to less all is fine here as well.

It's just that somehow "script.pl -p . /tmp/my.log | less +F" remains empty and it's not general problem because this:

perl -e 'print "a line\n" x 10' | less +F

works.

Axel