That's a business culture thing. If you set the expectation, that it's okay to do that, people might do that. If you set the expectation that it's not okay, people shouldn't.

The equivalent C and Python are likely just as ugly (given that the equivalent python might be using a far too complex range statement or put it on a single line using : and ;, which is possible...).

That said, as bad code goes, that's not really that bad. The only non-standard things are <> and $_, and if you don't know what those are, you really haven't done the minimum of learning about the language (<> might be somewhat exotic in that form depending on the codebase, but $_ is not something you can get way with not knowing about). In all, the major problem with it is that it's not commented and uses poor variable names, which is something every language needs to deal with. I would write it like so if I wanted it on a single line (but I would probably use 2-3 lines):

    # Report each duplicated line once
    my (%lineseen,@linedups); # This was required in yours too right? I assume you're using strict...
    while (<>) { (++$lineseen{$_}==2) && push(@linedups, $_); };

    # Report duplicate lines once
    my %lineseen;
    my @linedups = grep { ++$lineseen{$_}==2 } (<>);