It would appear to be an end of line thing. When your perl is in its own file, a comment extends to the end of the line. You can quite happyily put a semi-colon in a comment and perl just treats it as a comment.

Now extend this to the command line...

This works as expected:

perl -e 'printf "hello\n"'

This too works, as the comment is at the end:

perl -e 'printf "hello\n"; ## Comment'

This will not print anything because although the statements are separated by a semi-colon, perl has no newline to tell it where the comment ends:

perl -e '## Comment; printf "hello\n"'

This is how I entered a comment on the command line and got it to work as expected:

perl -e '## Comment; [RETURN WAS PRESSED HERE]
printf "Hello\n"'
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