The construct while(<>) is special as it iterates on the magical ARGV filehandle and sets $ARGV to the current filename, checking for definedness all the while. Whereas for(<>) just slurps ARGV and iterates over that (which is similar to the while construct, but not quite the same). Also you probably mean to use $_ instead of $ARGV (unless of course you've cheekily abused the global nature of *ARGV and used the scalar slot)¹. The while behaviour is well documented in Programming Perl (ver 3, page 82) and in the I/O Operators section of perlop.
HTH

_________
broquaint

¹ addendum - as Skeeve points out $ARGV holds the name of the current filename that is being iterated over, so is not in fact abuse of the scalar slot of *ARGV :)

update: added clarifications and fixed mistakes

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