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sub insert_after {
    my ($previous, $val) = @_;
...
    my ($previous, $val) = @_;
    return insert_after($previous->{prev}, $val);
}

sub display {
    my $curr = shift;
...
    }
    print "\n";
}

my $head = {data => 0, prev => undef, next => undef};
my $previous = $head;
...
my $new_node = insert_after ($head, 0.5);
$new_node = insert_before ($new_node->{next}, 0.7);
display($head);

$ perl  doubly_linked.pl
Current: 0      Previous:       Next: 1
...
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 5

my $head = {data => 0, prev => undef, next => undef};
my $tail = $head;
...
    chomp;
    $tail = insert_after($tail, $_);
}

#!/usr/bin/perl

...
$new_node = insert_before ($tail, 4.5);
display($head);

$ perl  doubly_linked.pl
...
Current: 3      Previous: 2     Next: 4
Current: 4      Previous: 3     Next: 4.5
Current: 4.5    Previous: 4     Next: 5