I don't think this is the complete answer as

read $IN1, my $buffer, -s $IN1;
++$counts{$_} for split( /^/m, $buffer );
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is faster than the for.

/dev/null
          Rate   for  read while
for   362612/s    --   -7%  -19%
read  389496/s    7%    --  -13%
while 449755/s   24%   15%    --

/usr/share/dict/words
        Rate   for  read while
for   4.73/s    --  -17%  -37%
read  5.73/s   21%    --  -24%
while 7.55/s   60%   32%    --

/etc/passwd
         Rate   for while  read
for   14434/s    --  -17%  -21%
while 17297/s   20%    --   -6%
read  18355/s   27%    6%    --

#!/usr/bin/perl
use strict;
use Benchmark qw( cmpthese );

foreach my $file qw ( /dev/null /usr/share/dict/words /etc/passwd ) {
    open my $IN1, '<', $file or die "could not open $file";
    my @list = <$IN1>;
    seek( $IN1, 0, 0 );
    print "$file\n";
    cmpthese(
        -5,
        {
            for => sub {
                seek( $IN1, 0, 0 );
                my %counts = ();
                ++$counts{$_} for <$IN1>;
                die unless keys %counts == @list;
            },
            while => sub {
                seek( $IN1, 0, 0 );
                my %counts = ();
                ++$counts{$_} while <$IN1>;
                die unless keys %counts == @list;
            },
            read => sub {
                seek( $IN1, 0, 0 );
                my %counts = ();
        read $IN1, my $buffer, -s $IN1;
                ++$counts{$_} for split( /^/m, $buffer );
                die unless keys %counts == @list;
            },
        }
    );
}
[download]