while ($line = <$fh>) {
    # ...
}

##</code><code>##

while (defined($line = <$fh>)) {
    # ...
}

##</code><code>##

my @a = qw( 1 2 3 4 5 );

while (my ($a, @a) = shift @a) {
	printf "Next value = $a\n";
	printf "Size of \@a = %d\n", 0 + @a;
}

# When run ... infinite loop!
# (Note that "Size of @a" shows the temporary @a, not the original)
Next value = 1
Size of @a = 0
Next value = 2
Size of @a = 0
Next value = 3
Size of @a = 0
Next value = 4
Size of @a = 0
Next value = 5
Size of @a = 0
Next value =
Size of @a = 0
Next value =
Size of @a = 0
Next value =