When you update your post: use strike tags and then put in correction. If you change the question mid-stream, posts to the original question are confusing. Make this update explicit.

And yes "unless" is just the "NOT of if". These are all the "same".

#!/usr/bin/perl
use strict;
use warnings;

my $x = 5;
print "x not 3\n" unless $x==3;

print "x not 3\n" if $x!=3;
print "x not 3\n" if !($x==3);
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Usually the "if version" is more clear. However in the above, since $x is seldom exactly 3, "unless" focuses on the usual action with Perl's ability to put the "action" first and the conditional last. For longer blocks, most often use some form of "if".