Since you said "array", you can use \$_ == \$array[-1]:

my @array= ( 1..5 );
for( @array ) {
    if(  \$_ == \$array[-1]  ) {
        print "Last element.\n";
    }
    print $_, $/;
}
__END__
1
2
3
4
Last element.
5
[download]

Update: ikegami asked about Data::Alias being able to confuse this test. Yes, that can be done, and one can even do it w/o such a module but it requires some unusual contortions and so probably isn't something to worry too much about:

my $five= 5;
*array= sub { \@_ }->( 1, 2, $five, 4, $five );
for( @array ) {
    if(  \$_ == \$array[-1]  ) {
        print "Last element.\n";
    }
    print $_, $/;
}
__END__
1
2
Last element.
5
4
Last element.
5
[download]