Python has 'pass', Bourne/Korn/Bash has ':', COBOL has (had?) NEXT-SENTENCE. But they were needed in those languages because (generally) of a lack of block delimiters, you shouldn't really need it in Perl. You can just have an empty block:
use strict;
use warnings;
my $x;
if (defined $x) {
# do nothing
}
else {
print "\$x is not defined\n"
}