It sounds like all you could possibly mean is the prompt where you type, before you hit enter, is where this is happening. If so, I can’t see how it has nothing anything to do with Perl. On OS X you can open the prefs for the Terminal with ⌘, then click the Advanced tab and make sure you have UTF-8 or something sane set. Mmmm… I just tested and found it to do what you describe, sort of.

moo@cow~>perl
while ( <STDIN> ) { print "Got: $_" }
^D
假借字 # <- pasted three ideograms
Got: 假借字
假借  # <- pasted same three and hit delete
Got: 假借
假借  # <- pasted same three and hit delete twice
Got: 假

So characters before the first appear to take two deletes to delete, with a phony/floating whitespace appearing between each contiguous characters, but they are actually deleted the first time, just erroneously displayed. This isn’t Perl’s fault, pretty sure, but some quirk in the Terminal. I would also like to know how to prevent it though I never saw it before. I messed around with switches -CSD too with no change in behavior.