pmount vs perl

After modifying debmirror, i copied it into a folder in my mounted portable hard drive.
The remaining space is quite large, so I put dapper & edgy as the target and start executing the script.
*bam*
myscript: /usr/bin/perl: bad interpreter: Permission denied

what the h###?!

I am absolutely sure I have no problem with permissions as the same script also copied into the parent directory of
a running ubuntu local ftp repository, and it works.

After checking every possible permission problem I encountered, I finally came into mount permission.
I mounted the portable hd using pmount (it's a usb-connected external hd) and it turnout that the default parameter
used by pmount include a noexec option, therefore, when I mounted it using the classic mount, the script worked.

Now, one question remain...
How come a perl script is considered a binary executable, while bash script is not?
Hmm...