Most folks who say Perl is ugly are talking about one of two things (or both): Sigils or Regular Expressions.
I think it is ugly because of its haphazard attitude to default behaviors. Sure the default is usually what you want, but if it isn't, there's no way to derive what you do want from principles, you just have to know it. The Perl documentation is littered with references to "magic" and so on. Whereas with (just for an example) Tcl, once you know the very basic principles, everything else can be determined logically from there.
Ah, yes, I suppose if Tcl is the pinnacle of beauty, then Perl would represent ugliness.
I've found only a few gotchas that really get me in Perl. Mostly its DWIM (Do What I Mean) philosophy really does what I mean. References, as I mentioned, are a source of trouble for me, but almost nothing else in Perl is.
I think it is ugly because of its haphazard attitude to default behaviors. Sure the default is usually what you want, but if it isn't, there's no way to derive what you do want from principles, you just have to know it. The Perl documentation is littered with references to "magic" and so on. Whereas with (just for an example) Tcl, once you know the very basic principles, everything else can be determined logically from there.