I think perlrun explains it pretty well:

       -x
       -x directory
            tells Perl that the program is embedded in a larger chunk 
+of unrelated ASCII text, such as in a mail message.  Leading garbage 
+will be discarded until the first line that starts
            with #! and contains the string "perl".  Any meaningful sw
+itches on that line will be applied.  If a directory name is specifie
+d, Perl will switch to that directory before running
            the program.  The -x switch controls only the disposal of 
+leading garbage.  The program must be terminated with "__END__" if th
+ere is trailing garbage to be ignored (the program
            can process any or all of the trailing garbage via the DAT
+A filehandle if desired).
[download]

Example:

cducdsgcsdcyvdsctydwcfvsc
rubbish
.....
more rubbish...
blah blah blah

#!/usr/bin/perl -w
use strict;
print "Hello World\n";

__END__
more garbage here ignored.
[download]

Output:

darren@barney:~/perlmonks$ perl -x garbage.pl
Hello World
[download]

Cheers,
Darren :)