As far as I can see, any scheme to hide the decrypted perl source is doomed to failure; for example:

# mv /usr/bin/perl /perl/bin/perl-
# vi /usr/bin/perl
...
# cat /usr/bin/perl
echo "ARGS=$@" > /tmp/out
# chmod 755 /usr/bin/perl
[download]

Update as appropriate depending on how the source gets passed to the perl binary.

Dave.