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"
}
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