use warnings;
use strict;

my @a2 = qw/1 2 2 3 3 3 4 4 4 4 5 6 7/;#list of to_visit
my @a1 = qw/1 2 3 4/;#list of visited
my %visited;
@visited{ @a1 } = 1;
@a2 = grep {not exists $visited{ $_ }} @a2;
print "list of to visit\n";
print_arrays(\@a2);
print "\nlist visited\n";
print_arrays(\@a1);

sub print_arrays
{
    my $x = shift;
    for my $item1 ( @$x )
    {
        print $item1,"\n";
    }
    print "\n";
}