They're two different things really - foreach takes a list and iterates over the given list, whereas while will keep looping until it's condition expression evaluates to false. In the first instance where you're using foreach, <FH> is evaluated in list context, as foreach evaluates it's provided arguments in list context, which will then read until it reaches an eof and then returns the lines that were read as a list. Whereas when you use a while, perl will auto-magically wrap your filehandle read with a check for definedness e.g

perl -MO=Deparse -e 'while(<>) { }'
while (defined($_ = <ARGV>)) {
    ();
}
-e syntax OK
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Then will continue looping until the filehandle reaches an eof, so only ever reads in a line at a time. See. perlsyn for more info on looping.
HTH

_________
broquaint