Good point!
So, e.g. by reversing the order of substatements inside the IF statement gives opposite result.

if(($check =~ m/YES/i) and (($line =~ m/$var1/i) || ($line =~ m/$var2/
+i))) { print "Inside if\n";}
else {print "Outside if\n";}
[download]

...unless ($check =~ m/Y/i) :)