Perl one-liner: deleting a line with pattern matching

Yes, it's the fact you're using single quotes. It means that $x is being interpreted literally.

Of course, you're also doing it very inefficiently, because you're processing each file multiple times.

If you're looking to remove lines abc12 to abc17 you can do this all in one go:

perl -n -i.bak -e 'print unless m/abc1[234567]/' filename