For anyone who's wondering: it prints the basename of the script it runs in, eg for /home/ap/t.pl it produces t.pl. The $^O stuff is to distinguish DOS vs Unix path separators (backslash vs slash).

It's not obfuscated at all, just overly dense, and not even really portable (slash works as a path separator on DOS as well; and it will fail on some systems like old MacOS and likely VMS). Here's how it should be done, readably, safely, and portably:

use File::Spec::Functions qw( rel2abs splitpath );
print +( splitpath rel2abs $0 )[2];
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Or, better yet, if you don't mind loading another module:

use File::Spec::Functions qw( rel2abs );
use File::Basename;
print basename rel2abs $0;
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But placing too much trust in $0 in the first place is a mistake anyway…