As soon as you create "buckets" instead of single hashes, the algorithm also works for multiple elements per key:

my @left = qw(1 2 3 1 1);
my %left;
for my $l (@left) {
    my $key = $l; # maybe you want some more complicated key than the 
+item itself
    $left{ $key } ||= [];
    push @{ $left{ $key } }, $l;
};

for my $r (@right) {
    my $key = $r; # maybe you want some more complicated key than the 
+item itself
    if (@{ $left{ $key }}) {
        my $found = shift @{ $left{ $key }};
        print "For $key, I found the pair ($found, $r).\n";
    } elsif (exists $left{ $key }) {
        print "For $key, there is at one item on the right side left o
+ver: $r.\n";
    } else {
        print "For $key, there is no item on the left side at all: $r.
+\n";
    };
};

# Now lets see if we have any values on the left side that did not pai
+r:
for my $l (keys %left) {
    my @leftover = @{ $left{ $l } };
    for (@leftover) {
        print "For $l, there was no corresponding element on the right
+ side ($_)\n";
    };
};
[download]

I haven't checked that code, but the approach is one I've used lots and lots when reconciling lists :)