The 1 is there because you have to supply some Perl code to -e, but in this case you don't care to do anything, you just want the side effects of the other switches. Using a 1 by itself as your "script" is just like having 1; on a line by itself in a script — it does nothing.

This can be used for such funny command lines as

perl -please textfile  # print the contents of the textfile
perl -deal  # fire up the debugger for interactive fiddling
[download]

These are equivalent to

perl -p -l -e '"ase"' textfile
perl -d -e '"al"'
[download]

where you simply have a string on a line by itself as the only statement in your "code", resulting in a no-op.